Allow images to be added to requested items

[fabian]

No description provided.

[PierreAnctil]

The UI should be similar to files upload of the supplied item

The following changes will be made:

• only images should be selectable
• The click on the drop zone will trigger a classic files selection (or files cannot be uploaded on mobile)
• The upload in the requested item creation should work (doesn't work on the supplied items probably because the supplied item is not yet created). Ajax upload will no longer be possible since the file(s) will need to be submitted with the requested item.

In my opinion, the last two points should also be changed at the supplied item level.