Export data from gridview to excel problem

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ASP.NET

MS-Excel

C#5.0

Hi friends, when i am trying to get the data from Gridview to excel data, when i am open the excel file, the file you are trying to open fileanme.xls is in a different format than specified by the file extension, when i click yes in that time only excel is open, but my client requirement is we need to directly open the excel without click yes button.
For this i tried different formats in my code xls,xlsx,csv................but not working

What I have tried:

protected void ExportToExcel(object sender, EventArgs e)
   {
       Response.Clear();
       Response.Buffer = true;
       Response.AddHeader("content-disposition", "attachment;filename=VIPadmVisitors.xls");
       Response.Charset = "";
       Response.ContentType = "application/vnd.ms-excel";
       Response.ContentEncoding = Encoding.Unicode;
       Response.BinaryWrite(Encoding.Unicode.GetPreamble());

       using (StringWriter sw = new StringWriter())
       {
           HtmlTextWriter hw = new HtmlTextWriter(sw);

           //To Export all pages
           gvVisitedVisitors.AllowPaging = false;
           //this.BindGrid();
           gvVisitedVisitors.Columns[1].Visible = false;
           gvVisitedVisitors.RenderControl(hw);

           //style to format numbers to string
           //string style = @"<style> .textmode { mso-number-format:\@; } </style>";
           string style = @"<style> td { mso-number-format:\@;} </style>";
           Response.Write(style);
           Response.Output.Write(sw.ToString());
           Response.Flush();
           Response.End();
       }
   }