@SuppressWarnings("unchecked")
    private <T> T readExtras(String name, JsonElement el, T input) throws AssetLoadException {
        ExtrasLoader loader = null;

        if (key != null) { // try to get the extras loader from the model key if available
            loader = key.getExtrasLoader();
        }
 
        if (loader == null) { // if no loader was found, use the default extras loader
            loader = getDefaultExtrasLoader();
        }

        if (loader == null) { // if default loader is not set or failed to instantiate, skip extras
            return input;
        }
           
        JsonElement extras = el.getAsJsonObject().getAsJsonObject("extras");
        if (extras == null) {
            return input;
        }

        try {
            return (T) loader.handleExtras(gltfLoader, name, el, extras, input);
        } catch (ClassCastException e) {
            throw new AssetLoadException("Extra loader " + loader.getClass().getName() + " for " + name + " is incompatible with type " + input.getClass(), e);
        }

    }
Returns the default extras loader.
@return the default extras loader.
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/CustomContentManager.java