Phpexcel create wrong file

this is my simple code to create an xlsx file and open it with openoffice

require(APPPATH.'/PHPExcel-1.8/Classes/PHPExcel.php');
require(APPPATH.'/PHPExcel-1.8/Classes/PHPExcel/Writer/Excel2007.php');
$this->load->library('excel');
$objPHPExcel = new PHPExcel();

$objPHPExcel->setActiveSheetIndex(0)
   ->SetCellValue('A1', 'Calendario Attivit');

$filename = "prova".".xlsx";
header("Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
header('Content-Disposition: attachment;filename="'.$filename.'"');
header("Cache-Control: max-age=0");
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');  
$objWriter->save('php://output');
exit;

the file is downloaded but when open it i get this:

someone can help me ?