bpo-37529: Add test for guessing extensions (GH-28243)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
index 4098a22644..392ddd2d5e 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -159,6 +159,15 @@ class MimeTypesTestCase(unittest.TestCase):
         # Poison should be gone.
         self.assertEqual(mimetypes.guess_extension('foo/bar'), None)
 
+    @unittest.skipIf(sys.platform.startswith("win"), "Non-Windows only")
+    def test_guess_known_extensions(self):
+        # Issue 37529
+        # The test fails on Windows because Windows adds mime types from the Registry
+        # and that creates some duplicates.
+        from mimetypes import types_map
+        for v in types_map.values():
+            self.assertIsNotNone(mimetypes.guess_extension(v))
+
     def test_preferred_extension(self):
         def check_extensions():
             self.assertEqual(mimetypes.guess_extension('application/octet-stream'), '.bin')