Use gdk_pixbuf_loader_new_with_mime_type for pixbuf thumbailer to prevent error 'Unrecognized file format'

Hi,

In /usr/share/thumbnailers I have heif-thumbnailer however, tumbler doesn't display the thumbnails but looking the output of tumblerd, the thumbnailer is registered.

Using dbus-monitor --session --monitor interface=org.freedesktop.thumbnails.Thumbnailer1 show me the error "Unrecognized file format".

The reason seems because the plugin pixbuf-thumbnailer use gdk_pixbuf_loader_new instead of gdk_pixbuf_loader_new_with_mime_type.

If you are interested I made a patch and here is a heif sample if you want to confirm the error.

Thank you