[BUGFIX] Fix invalid type hints in EXT:form's file upload converter

Due to invalid PHP type hints updating a previously uploaded file
reference in a mail form fails in the frontend.

Resolves: #83112
Releases: master, 8.7
Change-Id: Ibf98267a60f076b5654fb0d0d9181a4caeed49b3
Reviewed-on: https://review.typo3.org/54784


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>