how to export Datable into .xls only and download code .

i have write this code : 

ExcelPackage excel = new ExcelPackage();

var workSheet = excel.Workbook.Worksheets.Add(filnam);

//adding header column for the sheet

for (var i = 0; i < dt.Columns.Count; i++)

{

workSheet.Cells[1, i + 1].Value = dt.Columns[i].ColumnName;

}

//writing each row data

for (var j = 0; j < dt.Rows.Count; j++)

{

for (var i = 0; i < dt.Columns.Count; i++)

{

workSheet.Cells[j + 2, i + 1].Value = (dt.Rows[j][i]).ToString();

}

}

using (var memoryStream = new MemoryStream())

{

HttpContext.Current.Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";

HttpContext.Current.Response.AddHeader("content-disposition", "attachment; filename=" + filnam +".xls");

excel.SaveAs(memoryStream);

memoryStream.WriteTo(HttpContext.Current.Response.OutputStream);

HttpContext.Current.Response.Flush();

HttpContext.Current.Response.Close();

}

 

but  when it downloaded ,i opened it in MSOffice2007 it giving warning i.e.

"The file you are trying to open, 'filename.xls', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"

please solve this . Thnks