https://gist.github.com/diox/edf8713381db8fd004c0 fixes it. The culprit was the `af.update()` call in the form, because it shared the Addon instance with the form (thanks to related models caching introduced in 1.5), the `File` has a `post_save` signal that reloads the associated `Addon`, causing the instance stored on the form to be reseted completely.