[BUGFIX] Static addFlashMessage calls in extended file utility

ExtendedFileUtility still adds some flash messages statically. The
patch adapts to the new scheme and does some minor additinal
cleanup in the class for better type hintings in IDEs.

Change-Id: Ia75d2bccd9ed73a4055d1cf850b8f9703c59a76d
Resolves: #46773
Related: #44593
Releases: 6.1
Reviewed-on: https://review.typo3.org/19432
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn