public void registerExtensionLoader(String extensionName, ExtensionLoader loader) {
        extensionLoaders.put(extensionName, loader);
    }
Registers an extension loader for the given extension name.
For more information on extension please see glTF 2.0 extensions registry
https://github.com/KhronosGroup/glTF/blob/master/extensions/README.md

@param extensionName the name of the extension
@param loader        the Extension loader
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/GltfModelKey.java