i would like to show gridview data in microsoft office excel-2007,but it is automatically opening in Visual studio

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

public partial class Gridview_to_excel : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        bindgrid();
    }

    private void bindgrid()
    {
        SqlConnection con = new SqlConnection("Data Source=........;Initial Catalog=.....;Integrated Security=True");
        SqlDataAdapter da = new SqlDataAdapter("select * from gridview", con);
        DataSet ds = new DataSet();
        da.Fill(ds);
        GridView1.DataSource = ds;
        GridView1.DataBind();
        
    }
    protected void btnexcel_Click(object sender, EventArgs e)
    {
        Response.Clear();
        Response.AddHeader("content-dispotion", "attachment;filename=myexcelfile.xls");
        Response.ContentType = "applicaton/vnd.xls";
        StringWriter sw = new StringWriter();
        HtmlTextWriter tw = new HtmlTextWriter(sw);
        GridView1.RenderControl(tw);
        Response.Write(sw.ToString());
        Response.End();
    }
    public override void VerifyRenderingInServerForm(Control control)
    {
      
    }
}