gh-92455: Respect case-sensitive mimetype suffixes

Doc/library/mimetypes.rst

@@ -39,8 +39,8 @@ the information :func:`init` sets up.
    (e.g. :program:`compress` or :program:`gzip`). The encoding is suitable for use
    as a :mailheader:`Content-Encoding` header, **not** as a
    :mailheader:`Content-Transfer-Encoding` header. The mappings are table driven.
-   Encoding suffixes are case sensitive; type suffixes are first tried case
-   sensitively, then case insensitively.
+   Encoding suffixes are case-sensitive. Suffix mappings and type suffixes are
+   first tried case-sensitively, then case-insensitively.
 
    The optional *strict* argument is a flag specifying whether the list of known MIME types
    is limited to only the official types `registered with IANA
@@ -131,6 +131,8 @@ behavior of the module.
    is already known the extension will be added to the list of known extensions.
    Valid extensions are empty or start with a ``'.'``.
 
+   Registered lower-case extensions are matched case-insensitively.
+
    When *strict* is ``True`` (the default), the mapping will be added to the
    official MIME types, otherwise to the non-standard ones.
 
@@ -312,6 +314,8 @@ than one MIME-type database; it provides an interface similar to the one of the
       extension is already known, the new type will replace the old one. When the type
       is already known the extension will be added to the list of known extensions.
 
+      Registered lower-case extensions are matched case-insensitively.
+
       When *strict* is ``True`` (the default), the mapping will be added to the
       official MIME types, otherwise to the non-standard ones.
 

Lib/mimetypes.py

@@ -86,6 +86,9 @@ def add_type(self, type, ext, strict=True):
         is already known the extension will be added
         to the list of known extensions.
 
+        Registered lower-case extensions are matched
+        case-insensitively.
+
         If strict is true, information will be added to
         list of standard types, else to the list of non-standard
         types.
@@ -172,23 +175,33 @@ def guess_file_type(self, path, *, strict=True):
 
     def _guess_file_type(self, path, strict, splitext):
         base, ext = splitext(path)
-        while (ext_lower := ext.lower()) in self.suffix_map:
-            base, ext = splitext(base + self.suffix_map[ext_lower])
+        while True:
+            if ext in self.suffix_map:
+                suffix = self.suffix_map[ext]
+            elif (ext_lower := ext.lower()) in self.suffix_map:
+                suffix = self.suffix_map[ext_lower]
+            else:
+                break
+            base, ext = splitext(base + suffix)
         # encodings_map is case sensitive
         if ext in self.encodings_map:
             encoding = self.encodings_map[ext]
             base, ext = splitext(base)
         else:
             encoding = None
-        ext = ext.lower()
+        ext_lower = ext.lower()
         types_map = self.types_map[True]
         if ext in types_map:
             return types_map[ext], encoding
+        if ext_lower in types_map:
+            return types_map[ext_lower], encoding
         elif strict:
             return None, encoding
         types_map = self.types_map[False]
         if ext in types_map:
             return types_map[ext], encoding
+        if ext_lower in types_map:
+            return types_map[ext_lower], encoding
         else:
             return None, encoding
 
@@ -386,6 +399,9 @@ def add_type(type, ext, strict=True):
     is already known the extension will be added
     to the list of known extensions.
 
+    Registered lower-case extensions are matched
+    case-insensitively.
+
     If strict is true, information will be added to
     list of standard types, else to the list of non-standard
     types.

Lib/test/test_mimetypes.py

@@ -287,6 +287,50 @@ def test_case_sensitivity(self):
         eq(self.db.guess_file_type("foobar.tar.z"), (None, None))
         eq(self.db.guess_type("scheme:foobar.tar.z"), (None, None))
 
+    def test_suffix_map_case_sensitive_preferred(self):
+        self.db.suffix_map[".TEST-SUFFIX"] = ".tar.gz"
+        self.db.suffix_map[".test-suffix"] = ".tar.xz"
+        self.assertEqual(
+            self.db.guess_file_type("example.TEST-SUFFIX"),
+            ("application/x-tar", "gzip"),
+        )
+        self.assertEqual(
+            self.db.guess_file_type("example.test-suffix"),
+            ("application/x-tar", "xz"),
+        )
+
+    def test_added_types_case_sensitive_preferred(self):
+        self.db.add_type("text/x-test-uppercase-r", ".R")
+        self.db.add_type("text/x-test-lowercase-r", ".r")
+        self.assertEqual(
+            self.db.guess_file_type("example.R"),
+            ("text/x-test-uppercase-r", None),
+        )
+        self.assertEqual(
+            self.db.guess_file_type("example.r"),
+            ("text/x-test-lowercase-r", None),
+        )
+        self.db.add_type("text/x-test-uppercase-non-strict",
+                         ".NON-STRICT-EXT", strict=False)
+        self.db.add_type("text/x-test-lowercase-non-strict",
+                         ".non-strict-ext", strict=False)
+        self.assertEqual(
+            self.db.guess_file_type("example.NON-STRICT-EXT"),
+            (None, None),
+        )
+        self.assertEqual(
+            self.db.guess_file_type("example.non-strict-ext"),
+            (None, None),
+        )
+        self.assertEqual(
+            self.db.guess_file_type("example.NON-STRICT-EXT", strict=False),
+            ("text/x-test-uppercase-non-strict", None),
+        )
+        self.assertEqual(
+            self.db.guess_file_type("example.non-strict-ext", strict=False),
+            ("text/x-test-lowercase-non-strict", None),
+        )
+
     def test_default_data(self):
         eq = self.assertEqual
         eq(self.db.guess_file_type("foo.html"), ("text/html", None))

Misc/NEWS.d/next/Library/2026-04-20-01-24-22.gh-issue-92455.vXhmad.rst

@@ -0,0 +1,3 @@
+Fix :mod:`mimetypes` to prefer case-sensitive matches for suffix mappings and
+MIME type suffixes before falling back to case-insensitive matches.
+Contributed by Xiao Yuan.