diff -r 9f06cbb7962b Lib/email/message.py
--- a/Lib/email/message.py	Thu Mar 27 18:42:52 2014 -0400
+++ b/Lib/email/message.py	Thu Mar 27 18:55:13 2014 -0400
@@ -941,9 +941,7 @@
     @property
     def is_attachment(self):
         c_d = self.get('content-disposition')
-        if c_d is None:
-            return False
-        return c_d.lower() == 'attachment'
+        return False if c_d is None else c_d.content_disposition == 'attachment'
 
     def _find_body(self, part, preferencelist):
         if part.is_attachment:
diff -r 9f06cbb7962b Lib/test/test_email/test_message.py
--- a/Lib/test/test_email/test_message.py	Thu Mar 27 18:42:52 2014 -0400
+++ b/Lib/test/test_email/test_message.py	Thu Mar 27 18:55:13 2014 -0400
@@ -729,7 +729,8 @@
         self.assertTrue(m.is_attachment)
         m.replace_header('Content-Disposition', 'AtTachMent')
         self.assertTrue(m.is_attachment)
-
+        m.set_param('filename', 'abc.png', 'Content-Disposition')
+        self.assertTrue(m.is_attachment)
 
 
 class TestEmailMessage(TestEmailMessageBase, TestEmailBase):