private const string Boundary = "xYzZY";
//собираем форму в текстовом виде, параметры задаются в виде словаря .<имя параметра, значение>
 private static BinaryContainer GetPostData(IDictionary<string, string> Params, HttpFileCollectionBase files)
        {
            const string boundary = "--" + Boundary;

            BinaryContainer container = new BinaryContainer(Encoding.UTF8);

            //I do this because adding element to collection can produce unexpected side effects
            foreach (KeyValuePair<string, string> par in Params)
            {
                container.WriteLine(boundary);
                container.WriteLine(string.Format("Content-Disposition: form-data; name=\"{0}\"", par.Key));
                container.WriteLine();
                container.WriteLine(par.Value);
            }
            if (files != null)
                foreach (string sfile in files)
                {
                    var file = files[sfile];
                    container.WriteLine(boundary);
                    container.WriteLine(string.Format("Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"",
                                                      sfile,
                                                      file.FileName));
                    container.WriteLine(string.Format("Content-Type: {0}", file.ContentType));
                    container.WriteLine();
                    byte[] bytes = new byte[file.ContentLength];
                    file.InputStream.Read(bytes, 0, bytes.Length);
                    container.Write(bytes);
                    container.WriteLine();
                }


            container.WriteLine(boundary);
            return container;
        }

public static string PostRequestToServerWithSpecifiedHash(string ServiceURL,  IDictionary<string, string> Params,
            HttpFileCollectionBase fileCollection)

        {
            //url for routing testing. May be better put it to config
            string url = ServiceURL;
            HttpWebRequest req =  (HttpWebRequest)WebRequest.Create(url);
            req.KeepAlive = true;
            req.Method = "POST";

            var binaryContainer = GetPostData(Params, fileCollection);
            req.ContentLength = binaryContainer.Length;
            req.ContentType = "multipart/form-data;boundary=" + Boundary;
            
            Stream reqst = req.GetRequestStream(); // add form data to request stream
            binaryContainer.Flush(reqst);

            reqst.Flush();
            reqst.Close();

            HttpWebResponse res = (HttpWebResponse)req.GetResponse();
            Stream resst = res.GetResponseStream();

            StreamReader sr = new StreamReader(resst);
            string response = sr.ReadToEnd();

            return response;
        }