[BUGFIX] Change file_exists to is_file

Use is_file in a file only check, because a folder is
also a valid file_exists path!

Change-Id: Ia2dbf2f53a739dc6a8b63cecc555c712d70e6bc3
Reviewed-on: https://review.typo3.org/31409
Reviewed-by: Tim Spiekerkötter
Tested-by: Tim Spiekerkötter