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Backport of https://github.com/owncloud/core/pull/11368 to stable6 - since stable6 has no detection for SVG support this will just use the PNG icon instead of SVG ones.

I believe this is a better solution than backporting the whole SVG support stuff and potentially breaking a lot.
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.
Merge pull request #11371 from owncloud/backport-11368

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