protected function hashOfFiles($fileList)
    {
        // Get hash based on hash of each file
        $hash = '';
        foreach ($fileList as $file) {
            $absolutePath = Director::getAbsFile($file);
            if (file_exists($absolutePath)) {
                $hash .= sha1_file($absolutePath);
            } else {
                throw new InvalidArgumentException("Combined file {$file} does not exist");
            }
        }
        return sha1($hash);
    }