[BUGFIX] Fix fatal error in ExtendedFileUtility

The backport of #46595 to the 6.0 branch
did not take into account that the ExtendedFileUtility
misses the addFlashMessage method.

Replace the call with a static to fix the fatal
error.

Releases: 6.0
Resolves: #51672
Change-Id: I487b628c2282f5e6e3bd07643585c3323f1e4ffd
Reviewed-on: https://review.typo3.org/23549
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel