public function files($directoryName)
    {
        if (!$this->isDir($directoryName)) {
            throw new FileNotFoundException('Directory not found in the path [ ' . $directoryName . ' ].');
        }

        $files = new RecursiveIteratorIterator(
            new RecursiveDirectoryIterator($directoryName,
                RecursiveDirectoryIterator::SKIP_DOTS
            )
        );

        return array_filter(iterator_to_array($files), 'is_file');
    }