private function addFileSystemNode()
    {
        $builder = new TreeBuilder();
        $node = $builder->root('file_system');

        $node
            ->addDefaultsIfNotSet()
            ->children()
                ->scalarNode('directory')->defaultValue('%sulu.cache_dir%/preview')->end()
                ->scalarNode('extension')->defaultNull()->end()
                ->integerNode('umask')->defaultValue(0002)->end()
            ->end();

        return $node;
    }