如何在Alamofire4 URLRequestConvertible中使用multipart？

Question

如何使用multipart Alamofire URLRequestConvertible？

在Alamofire文档中

Alamofire.upload(
multipartFormData: { multipartFormData in
    multipartFormData.append(unicornImageURL, withName: "unicorn")
    multipartFormData.append(rainbowImageURL, withName: "rainbow")
},
to: "https://httpbin.org/post",
encodingCompletion: { encodingResult in
    switch encodingResult {
    case .success(let upload, _, _):
        upload.responseJSON { response in
            debugPrint(response)
        }
    case .failure(let encodingError):
        print(encodingError)
    }
}

但是URLRequestConvertible中没有多部分用法。