C# HttpClient以multipart/form-data形式 提交文件和其它參數

　　調用文件接口，需要一個上傳文件和一個Region參數，參考調用實例

 public async Task<WebApiResult> UploadFile(UploadFileModel info, IList<IFormFile> files)
        {
            try
            {
                var postContent = new MultipartFormDataContent();
                string boundary = string.Format("--{0}", DateTime.Now.Ticks.ToString("x"));
                postContent.Headers.Add("ContentType", $"multipart/form-data, boundary={boundary}");
                var requestUri = "/api/File/UploadFiles";
                if (files.Any())
                {
                    var stream = files[0].OpenReadStream();
                    //files為文件key, files[0].FileName 為文件名稱
                    postContent.Add(new StreamContent(stream, (int)stream.Length), "files", files[0].FileName);
                    //Region為請求文件接口需要的參數，根據調用接口參數而定
                    postContent.Add(new StringContent(info.Region), "Region");

                }
                var response = await _httpclient.PostAsync(requestUri, postContent);
                if (response.IsSuccessStatusCode)
                {
                    var responseStr = await response.Content.ReadAsStringAsync();
                    var responseObj = JsonConvert.DeserializeObject<WebApiResult>(responseStr);
                    return responseObj;
                }
                //WebApiResult為自定義反會值
                return new WebApiResult(ApiResultCode.Fail, "保存失敗");
            }
            catch (Exception ex)
            {
                throw new Exception("保存file異常");
            }

        }