Guess a file attachment's content type if application/octet-stream.

If a browser claims a file's content type is application/octet-stream,
get a second opinion by asking `file --mime-type`. This helps with some
browsers or server setups where that useless mimetype is being used,
preventing some files from being reviewable.

Fixes bug #2269
Reviewed at https://reviews.reviewboard.org/r/6016/