字符串参数太大时无法发送到API?

我正在调用一个web api来发布一些图片,有两个参数,

ActionId => int,
Images => [{"content":"base64","mime":"image/png"}]

当base64太大时,将从请求中省略该参数

 public static async Task<ImageResponseModel> UploadFile(List<ApiParameters> lstParameters, string url)
        {
            try
            {
                string boundary = "---boundaryb5dafaaadaad";
                MultipartFormDataContent form = new MultipartFormDataContent(boundary);

                foreach (var par in lstParameters)
                {
                    form.Add(new StringContent(par.ParameterValue), par.ParameterName);
                }

                HttpClient client = new HttpClient();
                client.DefaultRequestHeaders.Accept.Clear();
                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                client.DefaultRequestHeaders.Add("postman-token", "fe10d2c6-201e-ca7d-0d66-52d7f948f78f");
                client.DefaultRequestHeaders.Add("cache-control", "no-cache");
                client.DefaultRequestHeaders.Add("authorization", "the auth token here");


                HttpResponseMessage response = await client.PostAsync(ServerUrl + url, form);
                response.EnsureSuccessStatusCode();
                client.Dispose();
                string responseContent = await response.Content.ReadAsStringAsync();
                var apiResponse = JsonConvert.DeserializeObject<ImageResponseModel>(responseContent);
                return apiResponse;

            }
            catch (Exception ex)
            {
                Log.Report(ex);
                return null;
            }
        }

不能 StringContent 处理大块文本(+2MB图像,字符串长度超过6M个字符)?