how to prevent this message 'different format than specified by the file extension ' in excel file ... which is downloaded by c# code.

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ASP.NET

C#3.5

MS-Excel

hello friends.... im exprting an excel file in c# an my code is below... its downloading successfully . and when opening in ms excel , it shows a message that 'the file format different than specified by the file extension. how to prevent this message in c# (when downloading the file ). is there any solution? please guide . thanks in advance .

protected void Button1_Click1(object sender, EventArgs e)
{
try
{
Response.ClearContent();
Response.Buffer = true;
Response.AddHeader("content-disposition", string.Format("attatchment;filename={0}", "Dispatched_delivery_sample.xls"));
Response.Charset = "";
Response.ContentType = "application/ms-excel";
System.IO.StringWriter sw = new System.IO.StringWriter();
HtmlTextWriter htw = new HtmlTextWriter(sw);
GridView1.AllowPaging = false;
GridView1.HeaderStyle.BackColor = System.Drawing.Color.Orange;
for (int a = 0; a < GridView1.Rows.Count; a++)
{
GridViewRow row = GridView1.Rows[a];


}
GridView1.RenderControl(htw);
Response.Output.Write(sw.ToString());
Response.End();
}
catch
{


}
}