Когда я сохраняю файл полученный от HANDLERа и пытаюсь отрыть его, то получаю вот такое предупреждение:
"The file you trying to open, 'Users.xls', is in a different format than specified by the file extension. Verify that the file is not corrupt and is from a trusted source before opening the file. Do you want to open the file now?"
В чем проблема?

Большое спасибо

Вот код Handler:

using System.Web;
using System.Web.Services;
using System.IO;
using System.Data;
using System.Web.UI.WebControls;
using System.Web.UI;
using System.Text;

namespace WebApplication1
{
    public class Handler2 : IHttpHandler
    {
        public void ProcessRequest(HttpContext context)
        {        
            DataTable dt = new DataTable();
            GridView gv = new GridView();

            dt.Columns.Add("FirstName");
            dt.Columns.Add("LastName");
            dt.Columns.Add("Email");
            dt.Columns.Add("Phone");


            DataRow dr = dt.NewRow();
            dr["FirstName"] = "f";
            dr["LastName"] = "l";
            dr["Email"] = "e";
            dr["Phone"] = "p";
            dt.Rows.Add(dr);

            gv.DataSource = dt;
            ExportGridToExcel(gv, "Users.xls", context);
        }
        private void ExportGridToExcel(GridView grid, string filename, HttpContext context)
        {
            StringWriter sw  = new StringWriter();
            HtmlTextWriter htw = new HtmlTextWriter(sw);
            grid.AllowPaging = false;
            grid.AllowSorting = false;
            grid.DataBind();

            context.Response.Clear();
            context.Response.AddHeader("content-disposition", "attachment; filename=" + filename);
            context.Response.ContentType = "application/vnd.ms-excel";

            context.Response.ContentEncoding = Encoding.Unicode;
            context.Response.Charset = string.Empty;
            context.Response.BinaryWrite(Encoding.Unicode.GetPreamble());
            grid.RenderControl(htw);

            context.Response.Write(sw.ToString());

            context.Response.End();
        }
        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}