There's a quirky situation with browsers. If you want to map a form with a file upload field to an Ajax request, libraries (such as ExtJS and JQuery) help you out by creating an iframe. However, at least with ExtJS 4.1, HTTP errors get eaten somewhere.
So, it's never a good idea to return an HTTP error if you can avoid it. Instead, the best thing to do (at least with Ext) is to return success and a blob of JSON that lacks 'success: true'. Is there a way to install a provider that can map *all* errors, including things like missing mappings, for this purpose?
