[BUGFIX] Convert SVG files if fileExtension is explicitly set

If a "fileExtension" is given in the Image Processing of FAL
(e.g. via Fluid ViewHelper setting "fileExtension='png'"),
then a SVG image is now converted to the target file extension
as well.

This might be important for various use cases when dealing
with SVGs

Resolves: #75191
Releases: master, 10.4
Change-Id: I82fd2e7cccf15f845a7624a8dc9135bb06eaf145
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63611


Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>