[BUGFIX] Ensure correct return type for AbstractFile getters

Resolves: #76212
Releases: master, 7.6
Change-Id: I7b7ded93ff1d31f00f8d067f9697cd8f71305881
Reviewed-on: https://review.typo3.org/48195


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>