Apache Web Servers running on certain versions of Linux may serve the new Microsoft Document Formats incorrectly as zip files in IE6, 7 & 8.
Chrome, Firefox and Internet Explorer 9 (even when running in IE7 and IE8 testing modes) all will have no problem downloading files.
The solution is to add the following to your .htaccess file to correct the Mime Types:
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 AddType application/vnd.openxmlformats-officedocument.spreadsheetml.template xltx
Source: http://www.webdeveloper.com/forum/showthread.php?t=162526