Before commit 20523c61
PF_FILE used to open a "open file" filechooser dialog.
That commit, while adding the ability to open
a "save dialog" filechooser dialog using PF_FILENAME,
changed the behavior of the existing gimpfu PF_FILE type.

Restore the previous behavior and update the
built-in plug-ins using a save dialog to the new
PF_FILENAME.