diff -r 905d141e84da Lib/mimetypes.py
--- a/Lib/mimetypes.py	Sat Jan 25 15:32:06 2014 +0100
+++ b/Lib/mimetypes.py	Sat Jan 25 13:47:37 2014 -0300
@@ -104,7 +104,7 @@
         sensitive; type suffixes are first tried case sensitive, then
         case insensitive.
 
-        The suffixes .tgz, .taz and .tz (case sensitive!) are all
+        The suffixes .tgz, .taz and .tz are all
         mapped to '.tar.gz'.  (This is table-driven too, using the
         dictionary suffix_map.)
 
@@ -132,25 +132,22 @@
                 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() in self.suffix_map:
+            base, ext = posixpath.splitext(base + self.suffix_map[ext.lower()])
         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 -r 905d141e84da Lib/test/test_mimetypes.py
--- a/Lib/test/test_mimetypes.py	Sat Jan 25 15:32:06 2014 +0100
+++ b/Lib/test/test_mimetypes.py	Sat Jan 25 13:47:37 2014 -0300
@@ -77,6 +77,14 @@
                                           strict=True)
         self.assertEqual(exts, ['.g3', '.g\xb3'])
 
+    def test_case_sensitivity(self):
+        eq = self.assertEqual
+        #all suffixes are case insensitive
+        eq(self.db.guess_type("foo.HTML"), self.db.guess_type("foo.html"))
+        eq(self.db.guess_type("foo.TGZ"), self.db.guess_type("foo.tgz"))
+        #encodings are case sensitive
+        eq(self.db.guess_type("foo.tar.Z"), ("application/x-tar", "compress"))
+        eq(self.db.guess_type("foo.tar.z"), (None, None))
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
 class Win32MimeTypesTestCase(unittest.TestCase):