first off Great Plugin!
has become staple of my wordpress builds.
unfortunately i have found a small issue
the "file_gallery_cancel_file_deletion_if_attachment_copies" function will always cancels the deletion as it modifies the $file variable
to be relative to the uploads directory (e.g."yyyy/mm/filename.ext") rather than absolute.
if there are no copies it simply returns this altered $file variable causing the wordpress function to look for files relative to the posts.php file where the function resides.
simple fix is to use a copy of $file variable internaly and return the original at the bottom (and of course and empty string if there were copies).
keep up the good work!