public function getFile($fileName) {
        $files = $this->getFiles();
        $found = null;
        foreach ($files as $file) {
            if($file->getRelativePathname() === $fileName){
                if(!$file->isReadable()){
                    throw new RuntimeException(sprintf("The file '%s' could not be read.",$fileName));
                }
                $found = $file;
                break;
            }
        }
        return $found;
    }