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