Export Grid To Excel showing format error

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ASP.NET4

Hi All,

This is my code...

showing format extenton error. If i click yes after open excel file..

Please hele me.

Response.ClearContent();
Response.Buffer = true;
Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.AddHeader("content-disposition", "attachment;filename="Report.xls);
//Response.AddHeader("content-disposition", String.Format(@"attachment;filename={0}.xls", sheetName.Replace(" ", "_")));
Response.Charset = "";
Response.ContentType = "application/vnd.ms-excel";
StringWriter sw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(sw);
SalesGrid.AllowPaging = false;
SalesQuery(Convert.ToDateTime(TxtFromDt.Text), Convert.ToDateTime(TxtToDt.Text));
SalesGrid.RenderControl(hw);
Response.Output.Write(sw.ToString());
Response.Flush();
Response.End();