The "content type" should be set by using the existing property like in the next example:
httpResponse.ContentType = "application/vnd.ms-excel";
Other header like adding a file to download should be like in your example:
httpResponse.AddHeader("content-disposition", "attachment; filename=" + this.FileDownloadName);
And your object httpResponse should be used from your ASPX code behind, like in the next example:
HttpResponse httpResponse = HttpContext.Current.Response;