using(var client = new HttpClient())
using(var content = new MultipartFormDataContent())
{
  client.BaseAddress = new Uri("http://localhost:8080/");
  var fileContent = new ByteArrayContent(File.ReadAllBytes(fileName));
  fileContent.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
   {
     FileName = "foo.jpg"
   };

   content.Add(fileContent);
   FeedItemParams parameters = new FeedItemParams()
   {
     Id = "1234",
     comment = "Some comment about this or that."
   };
   content.Add(new ObjectContent<FeedItemParams>(parameters, new JsonMediaTypeFormatter()), "parameters");
   content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("multipart/form-data");

   var result = client.PostAsync("/api/ImageServices", content).Result;

public async Task<HttpResponseMessage> Post([FromBody]FeedItemParams parameters)

using (var client = new HttpClient())
{
    using (var content = new MultipartFormDataContent())
    {
        client.BaseAddress = new Uri("http://localhost:54711/");

        content.Add(new StreamContent(File.OpenRead(@"d:foo.jpg")), "foo", "foo.jpg");

        var parameters = new FeedItemParams()
        {
            Id = "1234",
            Comment = "Some comment about this or that."
        };
        content.Add(new ObjectContent<FeedItemParams>(parameters, new JsonMediaTypeFormatter()), "parameters");

        var result = client.PostAsync("/api/Values", content).Result;
    }
}

public async Task<HttpResponseMessage> PostFormData()
{
    // Check if the request contains multipart/form-data.
    if (!Request.Content.IsMimeMultipartContent())
    {
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    }

    string root = HttpContext.Current.Server.MapPath("~/App_Data");
    var provider = new MultipartFormDataStreamProvider(root);

    // Read the form data.
    await Request.Content.ReadAsMultipartAsync(provider);

    //use provider.FileData to get the file
    //use provider.FormData to get FeedItemParams. you have to deserialize the JSON yourself

    return Request.CreateResponse(HttpStatusCode.OK);
}