I have a Button with server-side OnClick handler, here are the codes
 protected void btnDownload_Click(object sender, ImageClickEventArgs e)   ***I PUT BREAKPOINT HERE***
    {
        try
        {
            string fileName = "ProductAvail_" + DateTime.Now.ToString("yyyyMMdd") + "_" + DateTime.Now.ToString("hmmsstt");
            string directory = Request.PhysicalApplicationPath + "Download/";
            string serverPath = HttpContext.Current.Request.Url.AbsoluteUri.Substring(0, HttpContext.Current.Request.Url.AbsoluteUri.LastIndexOf("/"));
            if (!System.IO.Directory.Exists(directory))
                System.IO.Directory.CreateDirectory(directory);
            DataTable table = (DataTable)ViewState["dtTemp2"];
            ExcelConvertor ex = new ExcelConvertor();
            ex.Convert(table, directory + fileName + ".xls");
            string url = "../download/download.aspx?fp=" + directory + fileName + ".xls";
            Response.Redirect(url);
        }
        catch (Exception exx)
        {
            lblMsg.Text = exx.Message;
        }
}
Code in download.aspx(page load event)
            string path = Request.QueryString["fp"].ToString();
            FileInfo file = new FileInfo(path);
            if (file.Exists)
            {
                
                System.IO.Stream iStream = null;
                // Buffer to read 10K bytes in chunk:
                byte[] buffer = new Byte[10000];
                // Length of the file:
                int length;
                // Total bytes to read:
                long dataToRead;
                // Identify the file to download including its path.
                string filepath = file.FullName; //"\\\\portnumbria\\Exceptions\\09012009_Exception_AccountOfficerUnitCasa.txt";
                // Identify the file name.
                string filename = System.IO.Path.GetFileName(filepath);
                try
                {
                    // Open the file.
                    iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open,
                                System.IO.FileAccess.Read, System.IO.FileShare.Read);
                    // Total bytes to read:
                    dataToRead = iStream.Length;
                    Response.ContentType = "application/octet-stream";
                    Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);
                    // Read the bytes.
                    while (dataToRead > 0)
                    {
                        // Verify that the client is connected.
                        if (Response.IsClientConnected)
                        {
                            // Read the data in buffer.
                            length = iStream.Read(buffer, 0, 10000);
                            // Write the data to the current output stream.
                            Response.OutputStream.Write(buffer, 0, length);
                            // Flush the data to the HTML output.
                            Response.Flush();
                            buffer = new Byte[10000];
                            dataToRead = dataToRead - length;
                        }
                        else
                        {
                            //prevent infinite loop if user disconnects
                            dataToRead = -1;
                        }
                    }
                }
                catch (Exception ex)
                {
                    // Trap the error, if any.
                    Response.Write("Error : " + ex.Message);
                }
                finally
                {
                    if (iStream != null)
                    {
                        //Close the file.
                        iStream.Close();
                    }
                    Response.Close();
                }