Код, который вы используете, просто отправляет файл ... вам нужно реализовать Async часть.
WebClient webClient = new WebClient();
webClient.UploadFileAsync(address, fileName);
webClient.UploadProgressChanged += WebClientUploadProgressChanged;
webClient.UploadFileCompleted += WebClientUploadCompleted;
...
void WebClientUploadProgressChanged(object sender, UploadProgressChangedEventArgs e)
{
Console.WriteLine("Download {0}% complete. ", e.ProgressPercentage);
}
void WebClientUploadCompleted(object sender, UploadFileCompletedEventArgs e)
{
// The upload is finished, clean up
}