diff -r d6aa3fa646e2 Lib/mimetypes.py
--- a/Lib/mimetypes.py	Fri Feb 21 18:30:53 2014 -0500
+++ b/Lib/mimetypes.py	Sat Mar 01 10:07:39 2014 -0700
@@ -68,6 +68,7 @@
         self.suffix_map = suffix_map.copy()
         self.types_map = ({}, {}) # dict for (non-strict, strict)
         self.types_map_inv = ({}, {})
+        self.canonical_map = canonical_map.copy()
         for (ext, type) in types_map.items():
             self.add_type(type, ext, True)
         for (ext, type) in common_types.items():
@@ -186,6 +187,9 @@
         Optional `strict' argument when false adds a bunch of commonly found,
         but non-standard types.
         """
+        type = type.lower()
+        if type in self.canonical_map:
+            return self.canonical_map[type]
         extensions = self.guess_all_extensions(type, strict)
         if not extensions:
             return None
@@ -339,7 +343,7 @@
 
 
 def init(files=None):
-    global suffix_map, types_map, encodings_map, common_types
+    global suffix_map, types_map, encodings_map, common_types, canonical_map
     global inited, _db
     inited = True    # so that MimeTypes.__init__() doesn't call us again
     db = MimeTypes()
@@ -354,6 +358,7 @@
     suffix_map = db.suffix_map
     types_map = db.types_map[True]
     common_types = db.types_map[False]
+    canonical_map = db.canonical_map
     # Make the DB a global variable now that it is fully initialized
     _db = db
 
@@ -374,6 +379,7 @@
     global encodings_map
     global types_map
     global common_types
+    global canonical_map
 
     suffix_map = {
         '.svgz': '.svg.gz',
@@ -542,6 +548,13 @@
         '.xul' : 'text/xul'
         }
 
+    # Certain types have well-known extensions.  These will be returned
+    # unconditionally from guess_extension.
+
+    canonical_map = {
+        'text/plain': '.txt',
+        }
+
 
 _default_mime_types()
 
diff -r d6aa3fa646e2 Lib/test/test_mimetypes.py
--- a/Lib/test/test_mimetypes.py	Fri Feb 21 18:30:53 2014 -0500
+++ b/Lib/test/test_mimetypes.py	Sat Mar 01 10:07:39 2014 -0700
@@ -3,6 +3,7 @@
 import mimetypes
 import sys
 import unittest
+import unittest.mock as mock
 
 from test import support
 
@@ -77,6 +78,12 @@
                                           strict=True)
         self.assertEqual(exts, ['.g3', '.g\xb3'])
 
+    def test_canonical_extensions(self):
+        eq = self.assertEqual
+        with mock.patch.dict(self.db.canonical_map, {'foo/bar': '.bar'}):
+            eq(self.db.guess_extension('foo/bar', strict=True), '.bar')
+            eq(self.db.guess_extension('foo/bar', strict=False), '.bar')
+
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
 class Win32MimeTypesTestCase(unittest.TestCase):