Filters whether to short-circuit moving the uploaded file after passing all checks.
If a non-null value is returned from the filter, moving the file and any related error reporting will be completely skipped.
(mixed) If null (default) move the file after the upload.
(string) An array of data for a single file.
(string) Filename of the newly-uploaded file.
(string) Mime type of the newly-uploaded file.