enviarArquivo.cs

        public static void EnviarArquivo(string url, string arquivo, string param, string contentType, NameValueCollection dadosFormulario) {
            string b = "---------------------------" + DateTime.Now.Ticks.ToString("x");
            byte[] bBytes = System.Text.Encoding.ASCII.GetBytes("\r\n--" + b + "\r\n");

            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
            req.ContentType = "multipart/form-data; boundary=" + b;
            req.Method = "POST";
            req.KeepAlive = true;

            Stream reqStream = req.GetRequestStream();

            string template = "Content-Disposition: form-data; name=\"{0}\"\r\n\r\n{1}";
            foreach (string key in dadosFormulario.Keys) {
                reqStream.Write(bBytes, 0, bBytes.Length);
                string itemForm = string.Format(template, key, dadosFormulario[key]);
                byte[] bytesItem = System.Text.Encoding.UTF8.GetBytes(itemForm);
                reqStream.Write(bytesItem, 0, bytesItem.Length);
            }
            reqStream.Write(bBytes, 0, bBytes.Length);

            string templateHeader = "Content-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"\r\nContent-Type: {2}\r\n\r\n";
            string header = string.Format(templateHeader, param, arquivo, contentType);
            byte[] bytesHeader = System.Text.Encoding.UTF8.GetBytes(header);
            reqStream.Write(bytesHeader, 0, bytesHeader.Length);

            FileStream fs = new FileStream(arquivo, FileMode.Open, FileAccess.Read);
            fs.CopyTo(reqStream);

            byte[] trail = System.Text.Encoding.ASCII.GetBytes("\r\n--" + b + "--\r\n");
            reqStream.Write(trail, 0, trail.Length);
            reqStream.Close();

            WebResponse resp = null;
            try {
                using (resp = req.GetResponse()) {
                    using (Stream respStream = resp.GetResponseStream()) {
                        StreamReader respReader = new StreamReader(respStream);
                        Console.WriteLine($"Resposta do Servidor: {respReader.ReadToEnd()}");
                    }
                }
            } catch (Exception ex) {
                Console.WriteLine($"Erro ao subir o arquivo {ex.Message} {ex.StackTrace}");
            }
        }