[BUGFIX] Fix absolute link generation for files

The method ContentRendererObject::forceAbsoluteUrl() fails
to generate absolute URLs for internal file resources even
if the flag "forceAbsoluteUrl" is activated.

Fix this by modifying the regexp that defines what is
accepted as an "already absolute link".

Resolves: #48529
Releases: master, 6.2
Change-Id: Iaf78d946f0f5cd35d175fe39159de72d18a7fa52
Reviewed-on: http://review.typo3.org/21036
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>