Fix SVG icons

FIXME: Ugly hack to prevent SVG of being returned if the SVG
provider is not enabled.
This is required because the preview system is designed in a
bad way and relies on opt-in with asterisks (i.e. image/*)
which will lead to the fact that a SVG will also match the image
provider.

Conflicts:
	lib/private/preview.php
Remove insane comment
Merge pull request #11611 from owncloud/fix-svg-master

Fix SVG icons