Filediffs have patched_sha256 and orig_sha256 attributes that are

hashes of their patched and original file contents. These get set for

non-binary file diffs the first time that they're loaded into the diff

viewer. Originally, these don't get set for binary file diffs.
We recently gave file attachments a sha256_checksum attribute which

is also a hash of its file contents. For binary file diffs, we can point

to it's corresponding file attachment's hash for both the patched file

and original file. Now, the SHAs will be set for binary file diffs, and

accessible through the file diff _sha256 attributes or the file

attachments' sha256_checksum.

Ran unit tests.
Accessed the patched_sha256 and orig_sha256 attributes for a

  binary file diff and saw it was the same as the file attachments'

  versions.