Add support for outputting Excel 2007 templates (.xltx)

[Daniel15]

Excel template files are identical to XLSX files, except with a different Content-Type (application/vnd.openxmlformats-officedocument.spreadsheetml.template.main+xml instead of application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml

[MarkBaker]

If those are the only differences, then the simplest way to output an Excel 2007 template doesn't require any code changes at all to the library:

// Redirect output to a client’s web browser (Excel2007)
header('Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.template');
header('Content-Disposition: attachment;filename="myfilename.xltx"');
header('Cache-Control: max-age=0');
// If you're serving to IE 9, then the following may be needed
header('Cache-Control: max-age=1');

// If you're serving to IE over SSL, then the following may be needed
header ('Expires: Mon, 26 Jul 1997 05:00:00 GMT'); // Date in the past
header ('Last-Modified: '.gmdate('D, d M Y H:i:s').' GMT'); // always modified
header ('Cache-Control: cache, must-revalidate'); // HTTP/1.1
header ('Pragma: public'); // HTTP/1.0

$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter->save('php://output');
exit;

[Daniel15]

I should have clarified: The [Content_Types].xml file is what needs the updated mimetype. I hacked around it by writing the .xlsx file with PHPExcel, then opening it with ZipArchive and modifying the [Content_Types].xml file directly.