fix(#3015137): point the field item at the moved file

File (Field) Paths kept the field item pointing at the pre-move file after moving it, so modules acting later in the same save read a path that no longer existed. Focal Point is the visible casualty: a focal point set on upload was lost on the first save and only stuck on the second. This points the field item at the moved file, so later hooks read the real location. Two kernel tests cover it, one of which forces the object swap that happens behind the widget. Credit to Nuuou for the 2018 report and to tvhung for finding the cause.