Fix creating file objects from a dragging pasteboard

Follows on from the `nsstringuti` branch - one more bug I found:

Dragging items into the QS window would make them appear as 'Unknown Object'. Now, I just resolve the `public.file-url` item, then we know they are files
(perhaps in reality for file objects, we should be storing an `NSURL` under `public.file-url` as opposed to a string path under `public.data`)

...ah well ;-)