One of our customers had a problem with Office 2007 files opening as zip files in IE8. The files opened as expected for Firefox users. In addition, everything works fine in the test environment (including IE8).

If you are experiencing the same problems, you should try the following.

1)
Check the MIME-types. The correct MIME-types are:

".manifest", "application/manifest"
".xaml", "application/xaml+xml",
".application",  "application/x-ms-application",
".deploy", "application/octet-stream"
".xbap", "application/x-ms-xbap"

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

We checked the production server, and the MIME-types were correct. So, we moved on to the next step.

2)
After emailing EPiServer support back and forth, it turns out that this is a bug: "Bug #31155: Office xml files (docx, pptx, ..) are treated as .zip files when downloaded in IE"

There are two solutions to this problem:
1 - Install Office 2007 on the server
2 - Install  MS Office Compatibility Pack for Word, Excel and Powerpoint 2007 on the server.

We did not want Office on the server, so we went for the compatibility pack solution. You can download the compability pack here.

Windows Server 2008 is NOT listed as a supported operating system, so we were a bit skeptical to install it in a production environment. But, we got information that it was sucessfully installed on a Windows 2008 server, so we decided to try it out.

3)
We installed the compability pack on the server, and voila :-) The Office 2007 files opens without any problems in IE8, Firefox, Opera, etc

Thanks to EPiServer Support for helping us find a workaround to the bug.