private function fileSize($fileSize)
    {
        if (isNumeric($fileSize)) {
            $decr = 1024;
            $step = 0;

            while (($fileSize / $decr) > 0.9) {
                $fileSize = $fileSize / $decr;
                $step++;
            }

            return round($fileSize, 2).' '.strtoupper($this->_prefix[$step]);
        } else {
            return 'NaN';
        }
    }