Export to Excel Default Save Type *.htm, *.html

Hi All,
 
I am using a simple way of creating excel from c#.net, I believe the below two lines are the important ones.
  HttpContext.Current.Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", fileName));
        HttpContext.Current.Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
I have ms office 2007, for which i guess the above code is fine.
I have searched at severl forums but could not find a solution.
Any help would be appreciated.
 
Thanks
menon
...