public static void registerDefaultExtrasLoader(Class<? extends ExtrasLoader> loader) {
        CustomContentManager.defaultExtraLoaderClass = loader;
    }
Sets the default extras loader used when no loader is specified in the GltfModelKey.

@param loader
           the default extras loader.
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/GltfLoader.java