[BUGFIX] Support SVG without XML prologue

If a SVG does not contains a XML prologue, TYPO3 indexes this file with
mimetype text/plain, which results in missing width and height in the
database. This patch adds a mapping for known filetypes to support
also files which mimetype can not detected automatically.

Resolves: #66445
Releases: master
Change-Id: I908e2f1bb95f9e59ebf23f79f09645afd6696ecd
Reviewed-on: http://review.typo3.org/38727


Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>