diff -r d6501421b86b -r 62b280b61de7 Lib/email/base64mime.py
--- a/Lib/email/base64mime.py	Wed Apr 16 18:33:39 2014 +0200
+++ b/Lib/email/base64mime.py	Thu Apr 17 10:50:30 2014 -0400
@@ -34,8 +34,8 @@
     ]
 
 
-from base64 import b64encode
-from binascii import b2a_base64, a2b_base64
+from base64 import b64encode, urlsafe_b64decode
+from binascii import b2a_base64
 
 CRLF = '\r\n'
 NL = '\n'
@@ -108,10 +108,12 @@
     """
     if not string:
         return bytes()
+    # Doing urlsafe b64decode is a little non-obvious, but a few email products
+    # seem to generate it. See #12489
     elif isinstance(string, str):
-        return a2b_base64(string.encode('raw-unicode-escape'))
+        return urlsafe_b64decode(string.encode('raw-unicode-escape'))
     else:
-        return a2b_base64(string)
+        return urlsafe_b64decode(string)
 
 
 # For convenience and backwards compatibility w/ standard base64 module
diff -r d6501421b86b -r 62b280b61de7 Lib/test/test_email/test_email.py
--- a/Lib/test/test_email/test_email.py	Wed Apr 16 18:33:39 2014 +0200
+++ b/Lib/test/test_email/test_email.py	Thu Apr 17 10:50:30 2014 -0400
@@ -4006,6 +4006,9 @@
         eq = self.assertEqual
         eq(base64mime.decode(''), b'')
         eq(base64mime.decode('aGVsbG8='), b'hello')
+        eq(base64mime.decode(
+           'QW5tZWxkdW5nIE5ldHphbnNjaGx1c3MgU_xkcmluZzNwLmpwZw=='),
+           b'Anmeldung Netzanschluss S\xfcdring3p.jpg')
 
     def test_encode(self):
         eq = self.assertEqual