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.