public <T> T readExtensionAndExtras(String name, JsonElement el, T input) throws AssetLoadException, IOException {
        T output = readExtension(name, el, input);
        output = readExtras(name, el, output);
        return output;
    }
Returns the default extras loader.
@return the default extras loader.
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/CustomContentManager.java