Nice one mate from the looks of things you know what you're doing, whereas I am just blundering about like a drunk elephant in a china shop
any news on this?
I'm having the same problem (mail form on frontend with UploadField to upload/sen an attachment).
Files are being uploaded but in the form action i get an empty file/upload array in $data.
I need to upload multiple files, so using FileField is not a suitable solution for me.
However i managed to disable the from-files-button like this:
I gave up and used "FileField" on the front end. Then let the relationship do its magic and save the stuff.
$fields->add(new FileField("Pic", "Advert Image"));
Seems to work OK. However, now that SS 3.0.3 is out they might have fixed it up. I had a feeling it was something to do with the object needing to be created before the picture could be uploaded (as you have to do in the backend).
Good luck and merry christmas.