Hi,
I have done searches and most post talk about the changes needs to be made on the server. I am pretty certain that has been done as I am able to attach/download both Docx & Xlsx file and the are store on my tables with the correct Mime Type:application/vnd.openxmlformats-officedocument.spreadsheetml.sheet. My app works perfectly with all 2003 MS version but nothing newer. I bet this has been posted but I have not been able to find it. I know we will be moving to 5.0.3 in 8 months but it seem that will not change this issue.
Apex 4.2.5
on 11g
Apache Tom Cat Server