Eu estou tentando fazer uma pós multipart formulário usando o HttpClient em c # e estou encontrando o seguinte código não funciona.

Importante:

var jsonToSend = JsonConvert.SerializeObject(json, Formatting.None, new IsoDateTimeConverter());
var multipart = new MultipartFormDataContent();
var body = new StringContent(jsonToSend, Encoding.UTF8, "application/json");

multipart.Add(body);
multipart.Add(new ByteArrayContent(File.ReadAllBytes("test.txt")), "test", "test.txt");

var httpClient = new HttpClient();
var response = httpClient.PostAsync(new Uri("http://localhost:55530"), multipart).Result;

Programa Completo :

namespace CourierMvc.Worker
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                Console.WriteLine("Hit any key to make request.");
                Console.ReadKey();

                try
                {
                    var request = new RestRequest(Method.POST)
                    {
                        Resource = "http://localhost:55530"
                    };

                    var json = new CourierMessage
                    {
                        Id = Guid.NewGuid().ToString(),
                        Key = "awesome",
                        From = "[email protected]",
                        To = new[] { "[email protected]", "[email protected]" },
                        Subject = "test",
                        Body = "body",
                        Processed = DateTimeOffset.UtcNow,
                        Received = DateTime.Now,
                        Created = DateTime.Now,
                        Sent = DateTime.Now,
                        Links = new[] { new Anchor { Link = "http://google.com" }, new Anchor { Link = "http://yahoo.com" } }
                    };

                    var jsonToSend = JsonConvert.SerializeObject(json, Formatting.None, new IsoDateTimeConverter());
                    var multipart = new MultipartFormDataContent();
                    var body = new StringContent(jsonToSend, Encoding.UTF8, "application/json");

                    multipart.Add(body);
                    multipart.Add(new ByteArrayContent(File.ReadAllBytes("test.txt")), "test", "test.txt");

                    var httpClient = new HttpClient();
                    var response = httpClient.PostAsync(new Uri("http://localhost:55530"), multipart).Result;

                }
                catch (Exception e)
                {
                    Console.WriteLine(e);
                }
            }
        }
    }
}

Eu realmente não tenho idéia porque não funciona. Eu pego o arquivo para postar no endpoint, mas o corpo (json) nunca chega lá. Estou fazendo algo errado?

Solicitação de Código do Lado do Servidor:

namespace CourierMvc.Controllers
{
    public class HomeController : Controller
    {
        //
        // GET: /Home/

        public ActionResult Index()
        {
            return Content("Home#Index");
        }


        [ValidateInput(false)]
        public ActionResult Create(CourierMessage input)
        {
            var files = Request.Files;

            return Content("OK");
        }

    }
}

Route Config:

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Create", id = UrlParameter.Optional }
    );

}