How to show progress bar with percentage while uploading WP8

Here is my code for uploading,

 byte[] buffer = PhoneApplicationService.Current.State["audiorecordedbytes"] as byte[];


            string url = ServiceOperations.Service_ImageUpload;
            var parameters = new Dictionary();
            parameters.Add("FileName", filename);
            parameters.Add("photo", buffer);


            this._currentPostTask = new HttpPostTask(url, parameters, this.OnPostCompleted);
            this._currentPostTask.Execute();

I want to show Progressbar with percentage, in WebClient we are having the ProgressChangedEvent for downloading not for uploading.Using HttpWebRequest we don't have such type of events.How can I show Progressbar with percentage in windows Phone 8.
Thanks in advance.