I figured out what the problem was ;)
I leftout a single header before sending, it seems that that has been the problem.
I forgot:
[code]
header('Content-Transfer-Encoding: binary');
[/code]
So all together, you'll need:
header('Content-type: application/vnd.openxmlformats-officedocument.wordprocessingml.document');
header('Content-Disposition: attachment; filename="report.docx"');
header('Content-Transfer-Encoding: binary');
Problem solved.