Wichert, it can't! Or if it can, I can't see where... :-/

Follow the path with me:

- You press the 'Save' button in the Archetypes form edit screen

- That form edit screen is a ControllerPageTemplate (base_edit.cpt) with validate_base.vpy as the unique validator

- validate_base.vpy calls BaseObject.validate() to check for errors

- BaseObject.validate() calls BasicSchema.validate() which in turn calls widget.process_form() for all fields

- We have a ReferenceField which uses a special reference widget, UploadReferenceWidget, which was designed to be a simple replacement for the default ReferenceWidget

- That widget receives a FileUpload object via REQUEST, proceeds with a 'File' creation and only returns its UID as the widget value

- Now that there's already a File in ZODB, the validation of another field fails, let's say 'title' was required but nothing was passed

- At this point the validation failed and you're back to the edit form screen with no chance to abort the transaction

Do you see the problem?

The point is: the only place where you can do additional stuff without changing any default Archetypes/CMFFormController settings is in the widget process_form() call, while at validation phase.

I'm still strongly arguing that CMFFormController would be the best place to have this fixed, but I can live with a change on Archetypes.

If you're worried about the 'transaction.abort()' being called by validate_base.vpy, I can suggest to fire a IObjectFailedValidation (or something like that) event there, that can be subscribed by third party code (the special widget, in this case).