var fileData = new Dictionary[string, string]() { { "Filename", fileName } };
var requestUri = Microsoft.Owin.Infrastructure.WebUtilities.AddQueryString(urlString, fileData);
var request = new HttpRequestMessage(HttpMethod.Post, requestUri);
var fileStreamContent = new StreamContent(fileStream);
fileStreamContent.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/pdf");
request.Content = fileStreamContent;
var response = new HttpClient().SendAsync(request, HttpCompletionOption.ResponseHeadersRead).Result;