diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
index a834826114614d..cf54984f9bbf68 100644
--- a/Lib/mimetypes.py
+++ b/Lib/mimetypes.py
@@ -187,15 +187,19 @@ 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
 
diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
index 2d618081521e10..7296f9ba0e1959 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -49,6 +49,18 @@ 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-r-script", ".R")
+        self.db.add_type("text/x-test-lowercase-r", ".r")
+        self.assertEqual(
+            self.db.guess_file_type("example.R"),
+            ("text/x-r-script", None),
+        )
+        self.assertEqual(
+            self.db.guess_file_type("example.r"),
+            ("text/x-test-lowercase-r", None),
+        )
+
     def test_default_data(self):
         eq = self.assertEqual
         eq(self.db.guess_file_type("foo.html"), ("text/html", None))
diff --git a/Misc/NEWS.d/next/Library/2026-04-20-01-24-22.gh-issue-92455.vXhmad.rst b/Misc/NEWS.d/next/Library/2026-04-20-01-24-22.gh-issue-92455.vXhmad.rst
new file mode 100644
index 00000000000000..7d251cae688ae8
--- /dev/null
+++ b/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.