core: Fix extension for album art URI
core: Fix extension for album art URI

Pelagicore/Media-Manager-Rygel-patched,Tarnyko/rygel,Tarnyko/rygel,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,jku/rygel,GNOME/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched