Recently I had an issue where various browsers on Windows desktops were saving docx, xslx, and pptx as zip files when downloaded from our linux web servers(apache in this case). The solution was to add extra mime-types to /etc/mime/types:
echo 'application/vnd.openxmlformats docx pptx xlsx' >> /etc/mime.types
And then restart apache:
sudo /etc/init.d/apache2 reload
This would also require mod_mime to be loaded, and is by default in Debian based systems. To verify the location of the mime.types file your server is using, the following commands may be helpful(replace the httpd.conf or mods conf directory with your distributions location).
grep -n mime.types /etc/apache2/mods-available/* grep -n mime.types /etc/apache2/httpd.conf
Linux desktops with Open Office had no such problems