The previous fix for missing file attachments broke once the f.close()

call was moved into the finally: block if the file attachment was

missing, due to the fact that the initial statement failed and f was

not a valid variable. This change fixes this by creating two separate

try/except blocks.
It also sandboxes the generation of text-based file attachment comment

thumbnails, which would also break the page if the file was missing.

Tested on a review request where both issues were hit. I was able to

get the page to load.