Show svg mime icons when no preview is available

Currently png images are used, which makes them look blurry on screen with high dpi

Fixed #7283

One thing I'm not sure about: to properly fallback to the mimetype icon js needs to know if a preview is available, in this PR I do that by returning a 404 for mimetypes that don't have an icon but I'm not sure if other apps/clients depend on that ajax call.

cc @jancborchardt @butonic @PVince81