Prevent hard to decipher error

There may still be a bug somewhere (possibly on the mobile device?) that is sending an attachment without a mime type header.

It's perfectly valid that `self.server_mime` may be `None` (look about 20 lines below in this file).

The error message sent to the client was `'NoneType' object has no attribute 'startswith'`, and it did not appear to be logged in couch.
No strong opinions on the first point. The second seems valid if this is screwing something up. Am sure you'll find out in QA