你那个 RestApi 是个啥？

如果是 System.Net.Http.HttpClient 上传 FormData 的话，应该像这样：

using System;
using System.Net.Http;

using (var httpClient = new HttpClient())
using (var httpRequest = new HttpRequestMessage(HttpMethod.Post, "/upload"))
{
    var formData = new MultipartFormDataContent();
    formData.Add(new StringContent("mywechat"), "wechat");
    formData.Add(new StringContent("mypicture1,mypicture2"), "pictures[]");
    formData.Add(new StringContent("mytitle"), "title");
    httpRequest.Content = formData;

    using (var httpResponse = await httpClient.SendAsync(httpRequest))
    {
        httpResponse.EnsureSuccessStatusCode();
    }
}

P.S. 头一次见到这么奇葩的接口字段。