gh-92455: Respect case-sensitive mimetype suffixes

Lib/mimetypes.py

@@ -180,15 +180,19 @@ def _guess_file_type(self, path, strict, splitext):
             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 +287,30 @@ 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_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", 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 MIME type suffixes
+registered with :func:`mimetypes.add_type` before falling back to
+case-insensitive matches. Contributed by Xiao Yuan.