怎么在一般处理程序中下载图片文件

public static void TransferFile(string filepath)    {       // StreamWriter sw = new StreamWriter(@"E:/NDadmin/ndfile/2.txt", true, Encoding.UTF8);        try        {            FileStream filestream = new FileStream(filepath, FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.Read);         //   sw.WriteLine("文件完成读取");            long filesize = filestream.Length;         //   sw.WriteLine("查询大小完成");            //让客户端浏览器正确识别这个文件的类型和文件大小            string filename = System.IO.Path.GetFileName(filepath).ToLower();        //    sw.WriteLine("识别大小完成");            HttpContext.Current.Response.ContentType = "application/octet-stream";         //    sw.WriteLine("制定类型完成");            HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=\"" + HttpUtility.UrlEncode(filename, System.Text.Encoding.UTF8) + "\";");         //    sw.WriteLine("文件位置制定完成");            HttpContext.Current.Response.AddHeader("Content-Length", filesize.ToString());             //sw.WriteLine("长度制定完成");            //将文件中的数据发送到客户端            byte[] filebuffer = new byte[filesize];            //sw.WriteLine("获取文件数据");            filestream.Read(filebuffer, 0, (int)filesize);             //sw.WriteLine("发送ING");            HttpContext.Current.Response.BinaryWrite(filebuffer);             //sw.WriteLine("发送完成");            filestream.Close();              //sw.WriteLine("释放");        }        catch (Exception ex)        {            HttpContext.Current.Response.Write(ex.Message);            throw ex;                          //sw.WriteLine("报错了！" + ex.Message);              //sw.WriteLine("报错了！" + ex.TargetSite);        }        finally        {            // sw.Flush();            //sw.Close();        }    }