diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
index 4750408173e..1aa32467e27 100644
--- a/Lib/mimetypes.py
+++ b/Lib/mimetypes.py
@@ -141,25 +141,23 @@ class MimeTypes:
                 type = 'text/plain'
             return type, None           # never compressed, so encoding is None
         base, ext = posixpath.splitext(url)
-        while ext in self.suffix_map:
-            base, ext = posixpath.splitext(base + self.suffix_map[ext])
+        while (ext_lower := ext.lower()) in self.suffix_map:
+            base, ext = posixpath.splitext(base + self.suffix_map[ext_lower])
+        # encodings_map is case sensitive
         if ext in self.encodings_map:
             encoding = self.encodings_map[ext]
             base, ext = posixpath.splitext(base)
         else:
             encoding = None
+        ext = ext.lower()
         types_map = self.types_map[True]
         if ext in types_map:
             return types_map[ext], encoding
-        elif 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
-        elif 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 392ddd2d5ee..3477b18376a 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -33,6 +33,13 @@ def tearDownModule():
 class MimeTypesTestCase(unittest.TestCase):
     def setUp(self):
         self.db = mimetypes.MimeTypes()
+        
+    def test_case_sensitivity(self):
+        eq = self.assertEqual
+        eq(self.db.guess_type("foobar.HTML"), self.db.guess_type("foobar.html"))
+        eq(self.db.guess_type("foobar.TGZ"), self.db.guess_type("foobar.tgz"))
+        eq(self.db.guess_type("foobar.tar.Z"), ("application/x-tar", "compress"))
+        eq(self.db.guess_type("foobar.tar.z"), (None, None))
 
     def test_default_data(self):
         eq = self.assertEqual
diff --git a/Misc/NEWS.d/next/Library/2021-12-22-12-02-27.bpo-20392.CLAFIp.rst b/Misc/NEWS.d/next/Library/2021-12-22-12-02-27.bpo-20392.CLAFIp.rst
new file mode 100644
index 00000000000..8973c4d4331
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-12-22-12-02-27.bpo-20392.CLAFIp.rst
@@ -0,0 +1 @@
+Fix inconsistency with uppercase file extensions in :meth:`MimeTypes.guess_type`. Patch by Kumar Aditya.