错误提示:{"errcode":40004,"errmsg":"不合法的媒体文件类型"}

源代码:(按着帮助文件写) DefaultDingTalkClient client = new DefaultDingTalkClient("https://oapi.dingtalk.com/media/upload"); OapiMediaUploadRequest req = new OapiMediaUploadRequest(); req.Type = "image"; Top.Api.Util.FileItem fileItem = new Top.Api.Util.FileItem(@"D:.net4.0\程序图片\28706.png");

        req.Media = fileItem;
        OapiMediaUploadResponse response = client.Execute(req, Access_Token.UpdateAccessToken(CorpID, CorpSecret));
        Console.WriteLine(response.Body);

都 不能成功,能不能完善一下C#的帮助文件.