This file documents the Emacs MIME interface functionality.

Copyright (C) 1998,99,2000, 2002 Free Software Foundation, Inc.

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Emacs MIME
**********

This manual documents the libraries used to compose and display MIME messages.

This is not a manual meant for users; it's a manual directed at people who want to write functions and commands that manipulate MIME elements.

MIME is short for Multipurpose Internet Mail Extensions. This standard is documented in a number of RFCs; mainly RFC2045 (Format of Internet Message Bodies), RFC2046 (Media Types), RFC2047 (Message Header Extensions for Non-ASCII Text), RFC2048 (Registration Procedures), RFC2049 (Conformance Criteria and Examples).  It is highly recommended that anyone who intends writing MIME-compliant software read at least RFC2045 and RFC2047.

Interface Functions
*******************

The mail-parse library is an abstraction over the actual low-level libraries that are described in the next chapter.

Standards change, and so programs have to change to fit in the new mold.  For instance, RFC2045 describes a syntax for the Content-Type header that only allows ASCII characters in the parameter list.  RFC2231 expands on RFC2045 syntax to provide a scheme for continuation headers and non-ASCII characters.

The traditional way to deal with this is just to update the library functions to parse the new syntax.  However, this is sometimes the wrong thing to do.  In some instances it may be vital to be able to understand both the old syntax as well as the new syntax, and if there is only one library, one must choose between the old version of the library and the new version of the library.

The Emacs MIME library takes a different tack.  It defines a series of low-level libraries (rfc2047.el, rfc2231.el and so on) that parses strictly according to the corresponding standard.  However, normal programs would not use the functions provided by these libraries directly, but instead use the functions provided by the mail-parse library.  The functions in this library are just aliases to the corresponding functions in the latest low-level libraries.  Using this scheme, programs get a consistent interface they can use, and library developers are free to create write code that handles new standards.

The following functions are defined by this library:

 -- Function: mail-header-parse-content-type string
     Parse STRING, a Content-Type header, and return a content-type list in the following format:

          ("type/subtype"
           (attribute1 . value1)
           (attribute2 . value2)
           ...)

     Here's an example:

          (mail-header-parse-content-type
           "image/gif; name=\"b980912.gif\"")
          => ("image/gif" (name . "b980912.gif"))

 -- Function: mail-header-parse-content-disposition string
     Parse STRING, a Content-Disposition header, and return a content-type list in the format above.

 -- Function: mail-content-type-get ct attribute
     Returns the value of the given ATTRIBUTE from the content-type list CT.

          (mail-content-type-get
           '("image/gif" (name . "b980912.gif")) 'name)
          => "b980912.gif"

 -- Function: mail-header-encode-parameter param value
     Takes a parameter string PARAM=VALUE and returns an encoded version of it.  This is used for parameters in headers like Content-Type and Content-Disposition.

 -- Function: mail-header-remove-comments string
     Return a comment-free version of STRING.

          (mail-header-remove-comments
           "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)")
          => "Gnus/5.070027  "

 -- Function: mail-header-remove-whitespace string
     Remove linear white space from STRING.  Space inside quoted strings and comments is preserved.

          (mail-header-remove-whitespace
           "image/gif; name=\"Name with spaces\"")
          => "image/gif;name=\"Name with spaces\""

 -- Function: mail-header-get-comment string
     Return the last comment in STRING.

          (mail-header-get-comment
           "Gnus/5.070027 (Pterodactyl Gnus v0.27) (Finnish Landrace)")
          => "Finnish Landrace"

 -- Function: mail-header-parse-address string
     Parse an address string STRING and return a list containing the mailbox and the plaintext name.

          (mail-header-parse-address
           "Hrvoje Niksic <hniksic@srce.hr>")
          => ("hniksic@srce.hr" . "Hrvoje Niksic")

 -- Function: mail-header-parse-addresses string
     Parse STRING as a list of addresses and return a list of elements like the one described above.

          (mail-header-parse-addresses
           "Hrvoje Niksic <hniksic@srce.hr>, Steinar Bang <sb@metis.no>")
          => (("hniksic@srce.hr" . "Hrvoje Niksic")
               ("sb@metis.no" . "Steinar Bang"))

 -- Function: mail-header-parse-date string
     Parse a date STRING and return an Emacs time structure.

 -- Function: mail-narrow-to-head
     Narrow the buffer to the header section of the buffer.  Point is placed at the beginning of the narrowed buffer.

 -- Function: mail-header-narrow-to-field
     Narrow the buffer to the header under point.

 -- Function: mail-encode-encoded-word-region start end
     Encode the non-ASCII words in the region STARTto END.  For instance, Naïve is encoded as =?iso-8859-1?q?Na=EFve?=.

 -- Function: mail-encode-encoded-word-buffer
     Encode the non-ASCII words in the current buffer.  This function is meant to be called with the buffer narrowed to the headers of a message.

 -- Function: mail-encode-encoded-word-string string
     Encode the words that need encoding in STRING, and return the result.

          (mail-encode-encoded-word-string
           "This is naïve, baby")
          => "This is =?iso-8859-1?q?na=EFve,?= baby"

 -- Function: mail-decode-encoded-word-region start end
     Decode the encoded words in the region STARTto END.

 -- Function: mail-decode-encoded-word-string string
     Decode the encoded words in STRING and return the result.

          (mail-decode-encoded-word-string
           "This is =?iso-8859-1?q?na=EFve,?= baby")
          => "This is naïve, baby"

Currently, mail-parse is an abstraction over ietf-drums, rfc2047, rfc2045 and rfc2231.  These are documented in the subsequent sections.

Basic Functions
***************

This chapter describes the basic, ground-level functions for parsing and handling.  Covered here is parsing From lines, removing comments from header lines, decoding encoded words, parsing date headers and so on.  High-level functionality is dealt with in the next chapter (see Decoding and Viewing).

rfc2045
=======

RFC2045 is the ``main'' MIME document, and as such, one would imagine that there would be a lot to implement.  But there isn't, since most of the implementation details are delegated to the subsequent RFCs.

So rfc2045.el has only a single function:

 -- Function: rfc2045-encode-string parameter value
     Takes a PARAMETER and a VALUE and returns a PARAM=VALUE string.  VALUE will be quoted if there are non-safe characters in it.

rfc2231
=======

RFC2231 defines a syntax for the Content-Type and Content-Disposition headers.  Its snappy name is MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations.

In short, these headers look something like this:

     Content-Type: application/x-stuff;
      title*0*=us-ascii'en'This%20is%20even%20more%20;
      title*1*=%2A%2A%2Afun%2A%2A%2A%20;
      title*2="isn't it!"

They usually aren't this bad, though.

The following functions are defined by this library:

 -- Function: rfc2231-parse-string string
     Parse a Content-Type header STRING and return a list describing its elements.

          (rfc2231-parse-string
           "application/x-stuff;
           title*0*=us-ascii'en'This%20is%20even%20more%20;
           title*1*=%2A%2A%2Afun%2A%2A%2A%20;
           title*2=\"isn't it!\"")
          => ("application/x-stuff"
              (title . "This is even more ***fun*** isn't it!"))

 -- Function: rfc2231-get-value ct attribute
     Takes a list CT of the format above and returns the value of the specified ATTRIBUTE.

 -- Function: rfc2231-encode-string parameter value
     Encode the string PARAMETER=VALUE for inclusion in headers likes Content-Type and Content-Disposition.

ietf-drums
==========

drums is an IETF working group that is working on the replacement for RFC822.

The functions provided by this library include:

 -- Function: ietf-drums-remove-comments string
     Remove the comments from STRING and return the result.

 -- Function: ietf-drums-remove-whitespace string
     Remove linear white space from STRING and return the result. Spaces inside quoted strings and comments are left untouched.

 -- Function: ietf-drums-get-comment string
     Return the last most comment from STRING.

 -- Function: ietf-drums-parse-address string
     Parse an address STRING and return a list of the mailbox and the plain text name.

 -- Function: ietf-drums-parse-addresses string
     Parse STRING, containing any number of comma-separated addresses, and return a list of mailbox/plain text pairs.

 -- Function: ietf-drums-parse-date string
     Parse the date STRING and return an Emacs time structure.

 -- Function: ietf-drums-narrow-to-header
     Narrow the buffer to the header section of the current buffer.

rfc2047
=======

RFC2047 (Message Header Extensions for Non-ASCII Text) specifies how non-ASCII text in headers are to be encoded.  This is actually rather complicated, so a number of variables are necessary to tweak what this library does.

The following variables are tweakable:

 -- Variable: rfc2047-default-charset
     Characters in this charset should not be decoded by this library. This defaults to iso-8859-1.

 -- Variable: rfc2047-header-encoding-list
     This is an alist of header / encoding-type pairs.  Its main purpose is to prevent encoding of certain headers.

The keys can either be header regexps, or t.

The values can be either nil, in which case the header(s) in question won't be encoded, or mime, which means that they will be encoded.

 -- Variable: rfc2047-charset-encoding-alist
     RFC2047 specifies two forms of encoding---Q (a Quoted-Printable-like encoding) and B (base64).  This alist specifies which charset should use which encoding.

 -- Variable: rfc2047-encoding-function-alist
     This is an alist of encoding / function pairs.  The encodings are Q, B and nil.

 -- Variable: rfc2047-q-encoding-alist
     The Q encoding isn't quite the same for all headers.  Some headers allow a narrower range of characters, and that is what this variable is for.  It's an alist of header regexps and allowable character ranges.

 -- Variable: rfc2047-encoded-word-regexp
     When decoding words, this library looks for matches to this regexp.

Those were the variables, and these are the functions:

 -- Function: rfc2047-narrow-to-field
     Narrow the buffer to the header on the current line.

 -- Function: rfc2047-encode-message-header
     Should be called narrowed to the header of a message.  Encodes according to rfc2047-header-encoding-alist.

 -- Function: rfc2047-encode-region start end
     Encodes all encodable words in the region START to END.

 -- Function: rfc2047-encode-string string
     Encode STRING and return the result.

 -- Function: rfc2047-decode-region start end
     Decode the encoded words in the region START to END.

 -- Function: rfc2047-decode-string string
     Decode STRING and return the result.

time-date
=========

While not really a part of the MIME library, it is convenient to document this library here.  It deals with parsing Date headers and manipulating time.  (Not by using tesseracts, though, I'm sorry to say.)

These functions convert between five formats: a date string, an Emacs time structure, a decoded time list, a number of seconds, and a day number.

The functions have quite self-explanatory names, so the following just gives an overview of which functions are available.

     (parse-time-string "Sat Sep 12 12:21:54 1998 +0200")
     => (54 21 12 12 9 1998 6 nil 7200)

     (date-to-time "Sat Sep 12 12:21:54 1998 +0200")
     => (13818 19266)

     (time-to-seconds '(13818 19266))
     => 905595714.0

     (seconds-to-time 905595714.0)
     => (13818 19266 0)

     (time-to-day '(13818 19266))
     => 729644

     (days-to-time 729644)
     => (961933 65536)

     (time-since '(13818 19266))
     => (0 430)

     (time-less-p '(13818 19266) '(13818 19145))
     => nil

     (subtract-time '(13818 19266) '(13818 19145))
     => (0 121)

     (days-between "Sat Sep 12 12:21:54 1998 +0200"
                   "Sat Sep 07 12:21:54 1998 +0200")
     => 5

     (date-leap-year-p 2000)
     => t

     (time-to-day-in-year '(13818 19266))
     => 255

And finally, we have safe-date-to-time, which does the same as date-to-time, but returns a zero time if the date is syntactically malformed.

qp
==

This library deals with decoding and encoding Quoted-Printable text.

Very briefly explained, QP encoding means translating all 8-bit characters (and lots of control characters) into things that look like =EF; that is, an equal sign followed by the byte encoded as a hex string.  It is defined in RFC 2045.

The following functions are defined by the library:

 -- Command: quoted-printable-decode-region FROM TO &optional CODING-SYSTEM
     QP-decode all the encoded text in the region.  If CODING-SYSTEM is non-nil, decode bytes into characters with that coding-system.  It is probably better not to use CODING-SYSTEM; instead decode into a unibyte buffer, decode that appropriately and then interpret it as multibyte.

 -- Function: quoted-printable-decode-string STRING &optional CODING-SYSTEM
     Return a QP-encoded copy of STRING.  If CODING-SYSTEM is non-nil, decode bytes into characters with that coding-system.

 -- Command: quoted-printable-encode-region FROM TO &optional FOLD CLASS
     QP-encode all the region.  If FOLD is non-NIL, fold lines at 76 characters, as required by the RFC.  If CLASS is non-nil, translate the characters not matched by that regexp class, which should be in the form expected by SKIP-CHARS-FORWARD and should probably not contain literal eight-bit characters.  Specifying CLASS makes sense to do extra encoding in header fields.

     If variable MM-USE-ULTRA-SAFE-ENCODING is defined and non-nil, fold lines unconditionally and encode From  and - at the start of lines..

 -- Function: quoted-printable-encode-string string
     Return a QP-encoded copy of STRING.

base64
======

Base64 is an encoding that encodes three bytes into four characters, thereby increasing the size by about 33%.  The alphabet used for encoding is very resistant to mangling during transit.  See Base 64 Encoding (The Emacs Lisp Reference Manual).

binhex
======

Binhex is an encoding that originated in Macintosh environments. The following function is supplied to deal with these:

 -- Function: binhex-decode-region start end &optional header-only
     Decode the encoded text in the region START to END.  If HEADER-ONLY is non-nil, only decode the binhex header and return the file name.

uudecode
========

Uuencoding is probably still the most popular encoding of binaries used on Usenet, although Base64 rules the mail world.

The following function is supplied by this package:

 -- Function: uudecode-decode-region start end &optional file-name
     Decode the text in the region START to END.  If FILE-NAME is non-nil, save the result to FILE-NAME.

rfc1843
=======

RFC1843 deals with mixing Chinese and ASCII characters in messages.  In essence, RFC1843 switches between ASCII and Chinese by doing this:

     This sentence is in ASCII.
     The next sentence is in GB.~{<:Ky2;S{#,NpJ)l6HK!#~}Bye.

Simple enough, and widely used in China.

The following functions are available to handle this encoding:

 -- Function: rfc1843-decode-region start end
     Decode HZ-encoded text in the region START to END.

 -- Function: rfc1843-decode-string string
     Decode the HZ-encoded STRING and return the result.

mailcap
=======

As specified by RFC 1524, MIME-aware message handlers parse mailcap files from a default list, which can be overridden by the MAILCAP environment variable.  These describe how elements are supposed to be displayed.  Here's an example file:

     image/*; gimp -8 %s
     audio/wav; wavplayer %s

This says that all image files should be displayed with gimp, and that WAVE audio files should be played by wavplayer.

The mailcap library parses such files, and provides functions for matching types.

 -- Variable: mailcap-mime-data
     This variable is an alist of alists containing backup viewing rules for MIME types.  These are overridden by rules for a type found in mailcap files.  The outer alist is keyed on the major content-type and the inner alists are keyed on the minor content-type (which can be a regular expression).

     For example:

          (("application"
            ("octet-stream"
             (viewer . mailcap-save-binary-file)
             (non-viewer . t)
             (type . "application/octet-stream"))
            ("plain"
             (viewer . view-mode)
             (test fboundp 'view-mode)
             (type . "text/plain")))

 -- User Option: mailcap-default-mime-data
     This variable is the default value of mailcap-mime-data.  It exists to allow setting the value using Custom.  It is merged with values from mailcap files by mailcap-parse-mailcaps.

Although it is not specified by the RFC, MIME tools normally use a common means of associating file extensions with defualt MIME types in the absence of other information about the type of a file.  The information is found in per-user files ~/.mime.types and system mime.types files found in quasi-standard places.  Here is an example:

     application/x-dvi	dvi
     audio/mpeg		mpga mpega mp2 mp3
     image/jpeg		jpeg jpg jpe

 -- Variable: mailcap-mime-extensions
     This variable is an alist MIME types keyed by file extensions. This is overridden by entries found in mime.types files.

 -- User Option: mailcap-default-mime-extensions
     This variable is the default value of mailcap-mime-extensions. It exists to allow setting the value using Custom.  It is merged with values from mailcap files by mailcap-parse-mimetypes.

Interface functions:

 -- Function: mailcap-parse-mailcaps &optional path force
     Parse all the mailcap files specified in a path string PATH and merge them with the values from mailcap-mime-data.  Components of PATH are separated by the path-separator character appropriate for the system.  If FORCE is non-nil, the files are re-parsed even if they have been parsed already.  If PATH is omitted, use the value of environment variable MAILCAPS if it is set; otherwise (on GNU and Unix) use the path defined in RFC 1524, plus /usr/local/etc/mailcap.

 -- Function: mailcap-parse-mimetypes &optional path force
     Parse all the mimetypes specified in a path string PATH and merge them with the values from mailcap-mime-extensions. Components of PATH are separated by the path-separator character appropriate for the system.  If PATH is omitted, use the value of environment variable MIMETYPES if set; otherwise use a default path consistent with that used by mailcap-parse-mailcaps. If FORCE is non-nil, the files are re-parsed even if they have been parsed already.

 -- Function: mailcap-mime-info string &optional request
     Gets the viewer command for content-type STRING.  nil is returned if none is found.  Expects STRING to be a complete content-type header line.

     If REQUEST is non-nil it specifies what information to return.  If it is nil or the empty string, the viewer (second field of the mailcap entry) will be returned.  If it is a string, then the mailcap field corresponding to that string will be returned (print, description, whatever).  If it is a number, all the information for this viewer is returned.  If it is all, then all possible viewers for this type is returned.

 -- Function: mailcap-mime-types
     This function returns a list of all the defined media types.

 -- Function: mailcap-extension-to-mime extension
     This function returns the content type defined for a file with the given EXTENSION.

Decoding and Viewing
********************

This chapter deals with decoding and viewing MIME messages on a higher level.

The main idea is to first analyze a MIME article, and then allow other programs to do things based on the list of handles that are returned as a result of this analysis.

Dissection
==========

The mm-dissect-buffer is the function responsible for dissecting a MIME article.  If given a multipart message, it will recursively descend the message, following the structure, and return a tree of MIME handles that describes the structure of the message.

Handles
=======

A MIME handle is a list that fully describes a MIME component.

The following macros can be used to access elements from the HANDLE argument:

 -- Macro: mm-handle-buffer handle
     Return the buffer that holds the contents of the undecoded MIME part.

 -- Macro: mm-handle-type handle
     Return the parsed Content-Type of the part.

 -- Macro: mm-handle-encoding handle
     Return the Content-Transfer-Encoding of the part.

 -- Macro: mm-handle-undisplayer handle
     Return the function that can be used to remove the displayed part (if it has been displayed).

 -- Macro: mm-handle-set-undisplayer handle function
     Set the undisplayer function for the part to function.

 -- Macro: mm-handle-disposition
     Return the parsed Content-Disposition of the part.

 -- Macro: mm-handle-disposition
     Return the description of the part.

 -- Macro: mm-get-content-id id
     Returns the handle(s) referred to by ID, the Content-ID of the part.

Display
=======

Functions for displaying, removing and saving.  In the descriptions below, `the part' means the MIME part represented by the HANDLE argument.

 -- Function: mm-display-part handle &optional no-default
     Display the part.  Return nil if the part is removed, inline if it is displayed inline or external if it is displayed externally.  If NO-DEFAULT is non-nil, the part is not displayed unless the MIME type of HANDLE is defined to be displayed inline or there is an display method defined for it; i.e. no default external method will be used.

 -- Function: mm-remove-part handle
     Remove the part if it has been displayed.

 -- Function: mm-inlinable-p handle
     Return non-nil if the part can be displayed inline.

 -- Function: mm-automatic-display-p handle
     Return non-nil if the user has requested automatic display of the MIME type of the part.

 -- Function: mm-destroy-part handle
     Free all the resources used by the part.

 -- Function: mm-save-part handle
     Save the part to a file.  The user is prompted for a file name to use.

 -- Function: mm-pipe-part handle
     Pipe the part through a shell command.  The user is prompted for the command to use.

 -- Function: mm-interactively-view-part handle
     Prompt for a mailcap method to use to view the part and display it externally using that method.

Customization
=============

The display of MIME types may be customized with the following options.

 -- User Option: mm-inline-media-tests
     This is an alist where the key is a MIME type, the second element is a function to display the part inline (i.e., inside Emacs), and the third element is a form to be evaled to say whether the part can be displayed inline.

     This variable specifies whether a part can be displayed inline, and, if so, how to do it.  It does not say whether parts are actually displayed inline.

 -- User Option: mm-inlined-types
     This, on the other hand, says what types are to be displayed inline, if they satisfy the conditions set by the variable above.  It's a list of MIME media types.

 -- User Option: mm-automatic-display
     This is a list of types that are to be displayed ``automatically'', but only if the above variable allows it.  That is, only inlinable parts can be displayed automatically.

 -- User Option: mm-attachment-override-types
     Some MIME agents create parts that have a content-disposition of attachment.  This variable allows overriding that disposition and displaying the part inline.  (Note that the disposition is only overridden if we are able to, and want to, display the part inline.)

 -- User Option: mm-discouraged-alternatives
     List of MIME types that are discouraged when viewing multipart/alternative.  Viewing agents are supposed to view the last possible part of a message, as that is supposed to be the richest. However, users may prefer other types instead, and this list says what types are most unwanted.  If, for instance, text/html parts are very unwanted, and text/richtech parts are somewhat unwanted, then the value of this variable should be set to:

          ("text/html" "text/richtext")

 -- User Option: mm-inline-large-images-p
     When displaying inline images that are larger than the window, XEmacs does not enable scrolling, which means that you cannot see the whole image.  To prevent this, the library tries to determine the image size before displaying it inline, and if it doesn't fit the window, the library will display it externally (e.g. with ImageMagick or xv).  Setting this variable to t disables this check and makes the library display all inline images as inline, regardless of their size.

 -- User Option: mm-inline-override-p
     mm-inlined-types may include regular expressions, for example to specify that all text/.* parts be displayed inline.  If a user prefers to have a type that matches such a regular expression be treated as an attachment, that can be accomplished by setting this variable to a list containing that type.  For example assuming mm-inlined-types includes text/.*, then including text/html in this variable will cause text/html parts to be treated as attachments.

New Viewers
===========

Here's an example viewer for displaying text/enriched inline:

     (defun mm-display-enriched-inline (handle)
       (let (text)
         (with-temp-buffer
           (mm-insert-part handle)
           (save-window-excursion
             (enriched-decode (point-min) (point-max))
             (setq text (buffer-string))))
         (mm-insert-inline handle text)))

We see that the function takes a MIME handle as its parameter.  It then goes to a temporary buffer, inserts the text of the part, does some work on the text, stores the result, goes back to the buffer it was called from and inserts the result.

The two important helper functions here are mm-insert-part and mm-insert-inline.  The first function inserts the text of the handle in the current buffer.  It handles charset and/or content transfer decoding.  The second function just inserts whatever text you tell it to insert, but it also sets things up so that the text can be ``undisplayed' in a convenient manner.

Composing
*********

Creating a MIME message is boring and non-trivial.  Therefore, a library called mml has been defined that parses a language called MML (MIME Meta Language) and generates MIME messages.

The main interface function is mml-generate-mime.  It will examine the contents of the current (narrowed-to) buffer and return a string containing the MIME message.

Simple MML Example
==================

Here's a simple multipart/alternative:

     <#multipart type=alternative>
     This is a plain text part.
     <#part type=text/enriched>
     <center>This is a centered enriched part</center>
     <#/multipart>

After running this through mml-generate-mime, we get this:

     Content-Type: multipart/alternative; boundary="=-=-="

     --=-=-=

     This is a plain text part.

     --=-=-=
     Content-Type: text/enriched

     <center>This is a centered enriched part</center>

     --=-=-=--

MML Definition
==============

The MML language is very simple.  It looks a bit like an SGML application, but it's not.

The main concept of MML is the part.  Each part can be of a different type or use a different charset.  The way to delineate a part is with a <#part ...> tag.  Multipart parts can be introduced with the <#multipart ...> tag.  Parts are ended by the <#/part> or <#/multipart> tags.  Parts started with the <#part ...> tags are also closed by the next open tag.

There's also the <#external ...> tag.  These introduce external/message-body parts.

Each tag can contain zero or more parameters on the form parameter=value.  The values may be enclosed in quotation marks, but that's not necessary unless the value contains white space.  So filename=/home/user/#hello$^yes is perfectly valid.

The following parameters have meaning in MML; parameters that have no meaning are ignored.  The MML parameter names are the same as the MIME parameter names; the things in the parentheses say which header it will be used in.

type
     The MIME type of the part (Content-Type).

filename
     Use the contents of the file in the body of the part (Content-Disposition).

charset
     The contents of the body of the part are to be encoded in the character set speficied (Content-Type).

name
     Might be used to suggest a file name if the part is to be saved to a file (Content-Type).

disposition
     Valid values are inline and attachment (Content-Disposition).

encoding
     Valid values are 7bit, 8bit, quoted-printable and base64 (Content-Transfer-Encoding).

description
     A description of the part (Content-Description).

creation-date
     RFC822 date when the part was created (Content-Disposition).

modification-date
     RFC822 date when the part was modified (Content-Disposition).

read-date
     RFC822 date when the part was read (Content-Disposition).

size
     The size (in octets) of the part (Content-Disposition).

Parameters for application/octet-stream:

type
     Type of the part; informal---meant for human readers (Content-Type).

Parameters for message/external-body:

access-type
     A word indicating the supported access mechanism by which the file may be obtained.  Values include ftp, anon-ftp, tftp, localfile, and mailserver.  (Content-Type.)

expiration
     The RFC822 date after which the file may no longer be fetched. (Content-Type.)

size
     The size (in octets) of the file.  (Content-Type.)

permission
     Valid values are read and read-write (Content-Type).

Advanced MML Example
====================

Here's a complex multipart message.  It's a multipart/mixed that contains many parts, one of which is a multipart/alternative.

     <#multipart type=mixed>
     <#part type=image/jpeg filename=~/rms.jpg disposition=inline>
     <#multipart type=alternative>
     This is a plain text part.
     <#part type=text/enriched name=enriched.txt>
     <center>This is a centered enriched part</center>
     <#/multipart>
     This is a new plain text part.
     <#part disposition=attachment>
     This plain text part is an attachment.
     <#/multipart>

And this is the resulting MIME message:

     Content-Type: multipart/mixed; boundary="=-=-="

     --=-=-=

     --=-=-=
     Content-Type: image/jpeg;
      filename="~/rms.jpg"
     Content-Disposition: inline;
      filename="~/rms.jpg"
     Content-Transfer-Encoding: base64

     /9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRof
     Hh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/wAALCAAwADABAREA/8QAHwAA
     AQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQR
     BRIhMUEGE1FhByJxFDKBkaEII0KxwRVS0fAkM2JyggkKFhcYGRolJicoKSo0NTY3ODk6Q0RF
     RkdISUpTVFVWV1hZWmNkZWZnaGlqc3R1dnd4eXqDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ip
     qrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uHi4+Tl5ufo6erx8vP09fb3+Pn6/9oACAEB
     AAA/AO/rifFHjldNuGsrDa0qcSSHkA+gHrXKw+LtWLrMb+RgTyhbr+HSug07xNqV9fQtZrNI
     AyiaE/NuBPOOOP0rvRNE880KOC8TbXXGCv1FPqjrF4LDR7u5L7SkTFT/ALWOP1xXgTuXfc7E
     sx6nua6rwp4IvvEM8chCxWxOdzn7wz6V9AaB4S07w9p5itow0rDLSY5Pt9K43xO66P4xs71m
     2QXiGCbA4yOVJ9+1aYORkdK434lyNH4ahCnG66VT9Nj15JFbPdX0MS43M4VQf5/yr2vSpLnw
     5ZW8dlCZ8KFXjOPX0/mK6rSPEGt3Angu44fNEReHYNvIH3TzXDeKNO8RX+kSX2ouZkicTIOc
     L+g7E810ulFjpVtv3bwgB3HJyK5L4quY/C9sVxk3ij/xx6850u7t1mtp/wDlpEw3An3Jr3Dw
     34gsbWza4nBlhC5LDsaW6+IFgupQyCF3iHH7gA7c9R9ay7zx6t7aX9jHC4smhfBkGCvHGfrm
     tLQ7hbnRrV1GPkAP1x1/Hr+Ncr8Vzjwrbf8AX6v/AKA9eQRyYlQk8Yx9K6XTNbkgia2ciSIn
     7p5Ga9Atte0LTLKO6it4i7dVRFJDcZ4PvXN+JvEMF9bILVGXJLSZ4zkjivRPDaeX4b08HOTC
     pOffmua+KkbS+GLVUGT9tT/0B68eeIpIFYjB70+OOVXyoOM9+M1eaWeCLzHPyHGO/NVWvJJm
     jQ8KGH1NfQWhXSXmh2c8eArRLwO3HSv/2Q==

     --=-=-=
     Content-Type: multipart/alternative; boundary="==-=-="

     --==-=-=

     This is a plain text part.

     --==-=-=
     Content-Type: text/enriched;
      name="enriched.txt"

     <center>This is a centered enriched part</center>

     --==-=-=--

     --=-=-=

     This is a new plain text part.

     --=-=-=
     Content-Disposition: attachment

     This plain text part is an attachment.

     --=-=-=--

Charset Translation
===================

During translation from MML to MIME, for each MIME part which has been composed inside Emacs, an appropriate MIME charset has to be chosen.

If you are running a non-Mule XEmacs, or Emacs in unibyte mode (Deprecated!), this process is simple: if the part contains any non-ASCII (8-bit) characters, the MIME charset given by mail-parse-charset (a symbol) is used.  (Never set this variable directly, though.  If you want to change the default charset, please consult the documentation of the package which you use to process MIME messages.  See Various Message Variables (Message Manual), for example.)  If there are only ASCII characters, the MIME charset US-ASCII is used, of course.

In a normal (multibyte) Emacs session, a list of coding systems is derived that can encode the message part's content and correspond to MIME charsets (according to their mime-charset property).  This list is according to the normal priority rules and the highest priority one is chosen to encode the part.  If no such coding system can encode the part's contents, they are split into several parts such that each can be encoded with an appropriate coding system/MIME charset. (The part can only be split at line boundaries, though---if more than one MIME charset is required to encode a single line, it is not possible to encode the part.)  Note that this procedure works with any correctly-defined coding systems, not just built-in ones.  Given a suitably-defined UTF-8 coding system---one capable of encoding the Emacs charsets you use---it is not normally necessary to split a part by charset.

It isn't possible to do this properly in XEmacs/Mule.  Instead, a list of the Mule charsets used in the part is obtained, and the corresponding MIME charsets are determined by lookup in mm-mime-mule-charset-alist.  If the list elements all correspond to a single MIME charset, that is used to encode the part.  Otherwise, the part is split as above.

Conversion
==========

A (multipart) MIME message can be converted to MML with the mime-to-mml function.  It works on the message in the current buffer, and substitutes MML markup for MIME boundaries. Non-textual parts do not have their contents in the buffer, but instead have the contents in separate buffers that are referred to from the MML tags.

An MML message can be converted back to MIME by the mml-to-mime function.

These functions are in certain senses ``lossy''---you will not get back an identical message if you run MIME-TO-MML and then MML-TO-MIME.  Not only will trivial things like the order of the headers differ, but the contents of the headers may also be different. For instance, the original message may use base64 encoding on text, while MML-TO-MIME may decide to use quoted-printable encoding, and so on.

In essence, however, these two functions should be the inverse of each other.  The resulting contents of the message should remain equivalent, if not identical.

Standards
*********

The Emacs MIME library implements handling of various elements according to a (somewhat) large number of RFCs, drafts and standards documents.  This chapter lists the relevant ones.  They can all be fetched from http://quimby.gnus.org/notes/.

RFC822
STD11
     Standard for the Format of ARPA Internet Text Messages.

RFC1036
     Standard for Interchange of USENET Messages

RFC1524
     A User Agent Configuration Mechanism For Multimedia Mail Format Information

RFC2045
     Format of Internet Message Bodies

RFC2046
     Media Types

RFC2047
     Message Header Extensions for Non-ASCII Text

RFC2048
     Registration Procedures

RFC2049
     Conformance Criteria and Examples

RFC2231
     MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations

RFC1843
     HZ - A Data Format for Exchanging Files of Arbitrarily Mixed Chinese and ASCII characters

draft-ietf-drums-msg-fmt-05.txt
     Draft for the successor of RFC822

RFC2112
     The MIME Multipart/Related Content-type

RFC1892
     The Multipart/Report Content Type for the Reporting of Mail System Administrative Messages

RFC2183
     Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field