[BUGFIX] Filemtime / Filesize trigger warning

Adds check for file_exists before calling filemtime / filsize
to avoid warning message.

Resolves: #53243
Releases: 6.2, 6.1, 6.0
Change-Id: I0f7da0c3e1920ddcf7d2d427a48ddcca71a6a4a3
Reviewed-on: https://review.typo3.org/25542
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters