Most of the file attachment stuff has had a visual revamp. This change cleans

up the last vestiges of stuff in the review dialog and review boxes: namely, we

don't care so much about displaying both the filename and the caption, in favor

of a unified "display name" for the attachment. We've also deprecated the file

type icons just about everywhere, so this removes it from the review box too.
This also removes the now-unused file_attachment_thumbnail.html template.

Checked the display of file attachment comments everywhere and saw that they

  looked correct.
Ran unit tests.
Ran js-tests.