The file attachment revisioning stuff doesn't actually put any restrictions on

the file type that's used to replace an attachment. This was fine when just

displaying individual revisions, but the comparison views would break in fun

and interesting ways.
This change fixes up our existing attachment UIs to show an error message in

the case where the comparison won't work. This currently has to be done

individually for each different Review UI, which isn't ideal. I'll make sure

that we get an item added to our backlog to try to clean that up in the future.

Set up a file attachment with many different revisions, each of a different

  type. Verified that I saw the correct thing in each of the different Review

  UIs (which get selected based on the "orig" version being shown).
Ran unit tests.
Ran js-tests.