When rendering custom formats with the <source> tag (as an example),
it currently is not possible to specificy a target file extension (e.g. webp
as alternative).

A new argument "fileExtension" for the <f:image>, <f:media> and
<f:uri.media> is introduced to specify a custom output format for having
variants on image variants.

Resolves: #90416
Releases: master
Change-Id: I5fe2a60b968578e504cda851d2d5dd0f0eefe7e0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63289


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>