public function getFilename(string $key): string
    {
        if (!$this->validate($key)) {
            throw new \InvalidArgumentException('The key provided (' . $key . ') is not valid! It may contain only the following characters: "a-z", "0-9", ".", "/", "-" and "_".');
        }
        if ($this->filenameProtocol === null) {
            throw new \Exception('No filenameProtocol specified!');
        }
        return $this->filenameProtocol . '://' . $key;
    }