protected function getPathForPartialUpload(string $key) : string
    {
        list($actualKey, /* $partialUploadKey */) = explode(self::PARTIAL_UPLOAD_NAME_SEPARATOR, $key);

        $path = $this->uploadDir . DIRECTORY_SEPARATOR . $actualKey . DIRECTORY_SEPARATOR;

        if ( ! file_exists($path)) {
            mkdir($path);
        }

        return $path;
    }