Alamofire上传进度，总预期字节数翻倍

Question

我尝试使用Swift Alamofire上传JSON数据，我的快速代码显示如下：

    var billString = Mapper().toJSON(bill!)

    var request = NSMutableURLRequest(URL: NSURL(string: String(NSString(format: "%@bill/add", BASEURL))
    request.HTTPMethod = "POST"
    request.setValue("application/json", forHTTPHeaderField: "Content-Type")

    let data: NSData = try! NSJSONSerialization.dataWithJSONObject(billString, options: [])

    manager.upload(request, data: data)
        .authenticate(user: user, password: password)
        .progress({ (written, totalWritten, expectedWritten) -> Void in
            onProgress(totalWritten, expectedWritten)
        })
        .responseObject { (response: Response<BillWrapper, NSError>) -> Void in }

帐单包含一些字符串，日期和图像数据。上传过程工作正常，它总是会成功上传整个帐单，但进展似乎并非如此。第一次当totalWritten达到预期值时，预计Written将自己加倍，并且进度将持续到第二次totalWritten达到预期值才停止。任何人都知道什么可能导致这个问题？感谢。