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

Lib/mimetypes.py

@@ -172,23 +172,33 @@ 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
 

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.