<ext:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" EnableAjax="false">//<span style="font-family: Arial, Helvetica, sans-serif;">EnableAjax="false"最重要了</span>

        </ext:Button>

 protected void Button1_Click(object sender, EventArgs e)
    {
        try
        {
            string filepath = "E:\\123.xls";
            string filename = System.IO.Path.GetFileName(filepath);
            Response.Clear();
            Response.ClearHeaders();
            Response.Buffer = false;
            Response.ContentType = "application/octet-stream";
            //Response.ContentEncoding=System.Text.Encoding.GetEncoding("utf-8");
            Response.AddHeader("Content-Disposition", "attachment; filename=" + System.Web.HttpUtility.UrlEncode(filename, Encoding.UTF8));
            Response.WriteFile(filepath);
            Response.Flush();
            Response.End();
        }
        catch (Exception E)
        {
            //alert("没有找到您所要下载的数据！
");
        }
    }