These dialogs are still driven by some pretty ancient javascript code in

common.js. All the backend code had been changed to use the new resource

models, but the error handling callback hadn't been updated for the new

attributes. The result of this was that instead of seeing an error in the form,

users would just see an endless "Loading..." spinner.

Tried to upload a .png file instead of a diff and saw the error shown next to

  the 'diff' field.
Tried to submit the 'Add File' form without selecting a file and saw the

  error shown next to the 'path' field.