Filefield upload widget broken in combofield

Followup after #942310: Field form cannot be attached more than once which enabled field widgets to live in subparts of a form - notably enabling combofield.

The file upload widget works perfectly in a simple nested form (like the one in field_test_entity_nested_form), but currently not in the prototype combofield available at http://github.com/yched/combo.

A primary look while testing #942310: Field form cannot be attached more than once got me lost in the widget's not trivial value handling. I didn't want to hold that other patch on a side case, and preferred investigating in a separate issue. Might need some help from file widget specialists - quicksketch, effulgentsia ? :-)

To reproduce :
- download http://github.com/yched/combo, enable combo.module (this creates a test setup with all the entities, fields and instances required)
- go to node/add/test-combo and create the node
- upload a file in the first widget : no file gets uploaded