I'm getting this error also.
CI is reporting a file type of 'application/zip' which makes sense as the xlsx format is a compressed format (rename it to zip and you can open the contents).
I have added/replaced the following line to the mime types file (application/config/mimes.php):
'xlsx' => array('application/vnd.openxmlformats-officedocument.spreadsheetml.sheet','application/zip'),
and that works (for this browser at least!)