[BUGFIX] Skip link validation of RTE filelink

When a text is linked to a file using RTE link wizard,
linkvalidator will detect it as internal link.
The validation check for internal page when it is a file will always fail.

This patch just skip validation if it finds a link to the file.

TODO:
* Proper validation of the file need to be applied.

Resolves: #49286
Releases: 6.0, 6.1, 6.2
Change-Id: I1b4901e2ac311ae93442fef044e4f7d7ae415753
Reviewed-on: https://review.typo3.org/21534
Reviewed-by: Visay Keo
Tested-by: Visay Keo
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe