C#.NET 微信上传电子小票

HttpWebRequest 时，不认图片的Content-Type。Content-Type 实际是有传的。

报错内容：{"code":"PARAM_ERROR","message":"电子小票图片类型必须是image/png或者image/jpg，请检查表单字段file中的Content-Type属性"}

后又换成 HttpClient，又报：

{"code":"SIGN_ERROR","detail":{"detail":{"issue":"sign not match"},"field":"signature","location":"authorization","sign_information":{"method":"POST","sign_message_length":98,"truncated_sign_message":"POST\n/v3/marketing/shopping-receipt/shoppingreceipts\n1664429854\n3f0ecffa77a440359db1b52279f81be1\n\n","url":"/v3/marketing/shopping-receipt/shoppingreceipts"}},"message":"错误的签名，验签失败"}

先用POSTMAN 调试，居然是通的。然后把URL的HTTPS 去掉，再用 Fiddler 抓包。Fiddler 抓包内容：

Content-Disposition: form-data; name="file"; filename="xp20220928173716.png"
Content-Type: image/png

和代码对比，Content-Disposition:和 form-data; 间有空格。form-data; 与 name 间有空格。 name="file"; 和 filename 间有空格。

照着改代码。

fileTemplate = "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n";

之后就正常了。

fileTemplate 原始内容（无法上传的版本）：

string fileTemplate = "Content-Disposition:form-data;name=\"{0}\";filename=\"{1}\"\r\nContent-Type:{2}\r\n\r\n";

END.

API 具体报错异常：

catch (WebException webEx2)
                {
                    if (webEx2.Response != null)
                    {
                        HttpWebResponse hwr = webEx2.Response as HttpWebResponse;
                        string exRsp = string.Empty;
                        using (Stream stream = hwr.GetResponseStream())
                        {
                            StreamReader sr = new StreamReader(stream);
                            exRsp = sr.ReadToEnd();
                        }
                        //throw new Exception(exRsp);
                        MessageBox.Show(exRsp);
                        //return;
                    }
                    //throw webEx2;
                }

要从 WebException 的message 中找。

图样说明：

name和filename 后边一定要有双引号，不然微信也接收不了。