Fix Gwenview being unable to open OpenRaster (ora) files

Despite having an ORA image plugin in kimageformats, Gwenview was unable to open them. It instead preferred to open them as a zip file, which is technically right but UX-wise wrong.

There was an explicit workaround to stop this from happening for svgz files which are also ZIPs. We should instead check the supported image mime types from Qt, and use that instead of hardcoding specific mimetypes.