[BUGFIX] Clipboard: Check whether FAL object is an existing file

The clipboard now checks whether a FAL object is a valid and existing
file before creating an export link.

Resolves: #89430
Related: #88969
Releases: master, 9.5, 8.7
Change-Id: I12f221962df59fa032a5b06e59d11c16dd37351c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62005


Tested-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christoph Lehmann <christoph.lehmann@networkteam.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>