[BUGFIX] Solves PHP warning in FileAllowedTypesValidator

Resolves: #72890
Resolves: #73618
Releases: master, 7.6
Change-Id: Ida8a9ace681b93d1d66b0e06c893a83e549f8411
Reviewed-on: https://review.typo3.org/46787


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>