Update object's file property after moving uploaded file

The call to `$file->move($dir, $name)` returns a new File object instead of the previous `UploadedFile`.

If we don't set the object's property to be that new File object, then the object still holds the UploadedFile  with the temporary upload path which isn't correct anymore.

The PR fixes this by setting the correct property on the object after the file has been moved.