Why are docx, xlsx, pptx downloading from webserver as zip files?

The files are actually ZIPs of XML documents. Add this to your .htaccess in Apache

AddType application/vnd.ms-word.document.macroEnabled.12 docm
AddType application/vnd.openxmlformats-officedocument.wordprocessingml.document docx
AddType application/vnd.openxmlformats-officedocument.wordprocessingml.template dotx
AddType application/vnd.ms-powerpoint.template.macroEnabled.12 potm
AddType application/vnd.openxmlformats-officedocument.presentationml.template potx
AddType application/vnd.ms-powerpoint.addin.macroEnabled.12 ppam
AddType application/vnd.ms-powerpoint.slideshow.macroEnabled.12 ppsm
AddType application/vnd.openxmlformats-officedocument.presentationml.slideshow ppsx
AddType application/vnd.ms-powerpoint.presentation.macroEnabled.12 pptm
AddType application/vnd.openxmlformats-officedocument.presentationml.presentation pptx
AddType application/vnd.ms-excel.addin.macroEnabled.12 xlam
AddType application/vnd.ms-excel.sheet.binary.macroEnabled.12 xlsb
AddType application/vnd.ms-excel.sheet.macroEnabled.12 xlsm
AddType application/vnd.openxmlformats-officedocument.spreadsheetml.sheet xlsx
AddType application/vnd.ms-excel.template.macroEnabled.12 xltm

The server doesn't have proper mime types set up for the new ms file types

add the following to the /etc/mime.types file and restart apache and the problem should be fixed.

# Added by myname 2009-06-03
application/vnd.ms-word.document.macroEnabled.12                        docm
application/vnd.openxmlformats-officedocument.wordprocessingml.document docx
application/vnd.openxmlformats-officedocument.wordprocessingml.template dotx
application/vnd.ms-powerpoint.template.macroEnabled.12                  potm
application/vnd.openxmlformats-officedocument.presentationml.template   potx
application/vnd.ms-powerpoint.addin.macroEnabled.12                     ppam
application/vnd.ms-powerpoint.slideshow.macroEnabled.12                 ppsm   
application/vnd.openxmlformats-officedocument.presentationml.slideshow  ppsx
application/vnd.ms-powerpoint.presentation.macroEnabled.12              pptm
application/vnd.openxmlformats-officedocument.presentationml.presentation       pptx
application/vnd.ms-excel.addin.macroEnabled.12                          xlam
application/vnd.ms-excel.sheet.binary.macroEnabled.12                   xlsb
application/vnd.ms-excel.sheet.macroEnabled.12                          xlsm
application/vnd.openxmlformats-officedocument.spreadsheetml.sheet       xlsx   
application/vnd.ms-excel.template.macroEnabled.12                       xltm   
application/vnd.openxmlformats-officedocument.spreadsheetml.template    xltx