Surround attachment filename in quotes

Certain browsers (Firefox) will truncate the filename if there's a space. Ideally we should do a bunch of fancy unicode handling (check out the headers sent by Gmail for a filename with utf-8 in it...) but this is a good solution for the short term.

There are no tests for this; there should probably be tests for this. I need to use this patch ASAP though, and don't have time to write a test suite for `BinaryResult` at the moment.

cc @xthexder