调用httpClient.PostAsync返回null

    [HttpPost]
    public IHttpActionResult Post()
    {

        string _endpoint = "https://someurl.com/api/v1/models?auth_token=mytoken";
        var httpContext = (System.Web.HttpContextWrapper)Request.Properties["MS_HttpContext"];
        string upload_id = httpContext.Request.Form["upload_id"];
        string filename =  httpContext.Request.Form["filename"];
        string filesize = "1000";


        //return this.Ok<string>(upload_id + " " + filename);         


        var content = new FormUrlEncodedContent(new[] 
        {
            new KeyValuePair<string, string>("upload_id", upload_id),
            new KeyValuePair<string, string>("filename", filename),
            new KeyValuePair<string, string>("filesize", filesize)
        });  

        using (var httpClient = new HttpClient())
        {
            var response = httpClient.PostAsync(_endpoint, content).Result;
            return Json(JsonConvert.DeserializeObject(response.Content.ReadAsStringAsync().Result));
        }



    }

然后,我通过ajax打电话给客户端以获取数据：

$.ajax({
        url: '/api/tws',
        type: 'POST',
        data: { 'file': "EX-IGES.IGS", 'upload_id': "eb550576d2" },
        success: function (response) { 
                 console.log('response',response);
                 }
 });

但是,它总是返回null.我已经验证了API调用的工作原理,并且一切正确.我对C#有点陌生.

固定的ajax代码：

$.ajax({ url: '/api/tws', 
       type: 'POST', 
       data: { 'filename': "EX-IGES.IGS", 'upload_id': "eb550576d2" }, 
       success: function (response) { console.log('response',response); } 
 });