Fix a couple more issues with attachments with bogus mimetypes.
Review Request #7048 — Created March 12, 2015 and submitted
This fixes up a couple more crashes that happen when browsers and the 'file'
command give us a bogus mimetype.
Attached the given outlook .msg file in the bug and saw that everything worked
correctly.
Tool: Pyflakes Processed Files: reviewboard/attachments/mimetypes.py reviewboard/attachments/forms.py Tool: PEP8 Style Checker Processed Files: reviewboard/attachments/mimetypes.py reviewboard/attachments/forms.py