creating .xlsx file fro sql data

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C#

ASP.NET

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.IO;


public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        ShowData();
        string dbcon = ConfigurationManager.ConnectionStrings["cn"].ToString();
    }
    public override void VerifyRenderingInServerForm(Control control)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Excel2007.WorkBook workbook = new Excel2007.WorkBook();

        Response.Clear();
        Response.BufferOutput = false;
        string archiveName = "customer.xlsx";
        Response.ContentType = "application/zip";
        Response.AddHeader("content-disposition", "filename=" + archiveName);
        Response.Charset = "";
        workbook.WorkSheets.Add("FirstSheet");
        workbook.WorkSheets["FirstSheet"].Cells[0, 1].Value = "first";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        GridView1.AllowPaging = false; GridView1.DataBind();
        GridView1.HeaderRow.Style.Add("background-color", "#FFFFFF");
        GridView1.RenderControl(htw);
        Response.Write(sw.ToString());
        workbook.Save(Response.OutputStream);
        Response.End();
    }

    public void ShowData()
    {

        SqlDataAdapter adp = new SqlDataAdapter("select catname,catid,creationdate,pagename from quizcats  ", ConfigurationManager.ConnectionStrings["cn"].ConnectionString);
        DataSet ds = new DataSet();
        adp.Fill(ds);
        GridView1.DataSource = ds;
        GridView1.DataBind();
    }
}

the .xls file can be easily created with this code but whenever i change the extension to .xlsx then the file can't be open in excel 2007..please point out the error and suggest me something with the same coding..