将文件上传到Google云端硬盘时出错403错误

Question

我正在将文件上传到Google云端硬盘（https://developers.google.com/drive/manage-uploads） 当我点击上传按钮时，我收到错误：“403 Forbidden” 我的代码：

string access_token = "ya29.1.AADtN_USZWM44X_WIxgAr-f6NcdQ-XG8TEturcGXRADzdeVvXZhSVR8d6Qf98veHqy3ZGLQ";
        string result = "";
        byte[] buffer = uploader.FileBytes;
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create("https://www.googleapis.com/upload/drive/v2/files?uploadType=media");

        req.Method = "POST";
        req.ContentType = "image/jpeg";
        req.ContentLength = buffer.Length;
        req.Headers["Authorization"] = "Bearer "+access_token;

        Stream strm = req.GetRequestStream();
        strm.Write(buffer, 0, buffer.Length);
        strm.Close();

        HttpWebResponse webResponse = (HttpWebResponse)req.GetResponse();//Get error here
        if (webResponse.StatusCode == HttpStatusCode.OK)
        {
            Stream responseStream = webResponse.GetResponseStream();
            StreamReader responseStreamReader = new StreamReader(responseStream);
            result = responseStreamReader.ReadToEnd();//parse token from result
        }
        lblMessage.Text = result;
        webResponse.Close();

请帮助我！