[BUGFIX] ResourceStorage::SIGNAL_PreFileAdd should be triggered earlier

The PreFileAdd signal should be triggered before the file exist check.
So the name and/or content can be manipulated before the target filename
is checked.

Change-Id: I0bbe32a4efa395a99278c480210310bd7466f259
Fixes: #56394
Releases: 6.2
Reviewed-on: https://review.typo3.org/28099
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring