The `get_for_filediff` method for getting file attachments attached to diffs had a few major issues that were preventing it from functioning correctly. First, it was querying solely based on the repository, file path, and revision. This meant we could theoretically end up returning `FileAttachment` objects which were attached to a different review request than the given `FileDiff`. While our database schema theoretically would allow for a single FileAttachment to be attached to multiple review requests, in practice everything assumes that it's more of a foreign key relationship than a many to many relationship. Second, when a review request is created with commit history, there are actually multiple FileDiffs that all go into a single changed file. At the very least, we'll have a FileDiff for the commit and a FileDiff for the cumulative diff. When we're posting changes from RBTools, we attach the FileAttachment to the FileDiff corresponding to the commit. This meant that even though the file was uploaded, it wasn't visible in the diff viewer because we'd usually be looking at the cumulative diff. This change makes it so `FileAttachment.objects.get_for_filediff` will first try the simple case, and if that isn't found, it will check to see if the DiffSet is created with commit history. If that's the case, we'll look for the most recent FileDiff object corresponding to the given file and use that instead. Finally, we mostly were relying on this function to raise `ObjectDoesNotExist`, but we also had a case where it could return `None`. I've changed it to always return `None` when there are no matching file attachments. Testing Done: - Made some queries to filediff API resources and saw that the serialized attachment fields no longer contained links to file attachments connected to other review requests. - Posted a change with RBTools that included binary files. Saw that I could now see those files in the diff viewer. - Ran unit tests.