A working version of a Backbone view for file uploads (the one that you see when you click Update -> Add File). Previously that popup is used gReviewRequest hack. Now it's a standalone view, as that view is also needed for attachment revisioning.
Side note: The only other thing that needs gReviewRequest hack is "Update Diff" popup. Once someone writes a view for that, gReviewRequest can be removed.

Tested:
* Uploading new attachments.
* Error when no attachment is specified.
* Multiple errors returned from server side (see screenshots).