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.
Good point. My thought was that returning `None` is the same as returning `False` when the value is being tested for truthiness, but allows the caller to check if the answer was actually "unknown" rather than simply "yes" or "no" if that becomes important. I guess at this point it's an unused feature and more code to maintain, so there's definitely that tradeoff. I can change it if you think it's important.

Also, given the discussion happening in email, it seems like this function should probably also look at the file name extension if the server does not provide a mime type.