.docx, .pptx, .xlsx open as compressed folder in IE8.

A quick one today.

I had a case where the customer reported that IIS 6.0 returned Office 2007 / 2010 documents (.docx, .pptx, .xlsx, etc.) as compressed (.zip) files.
Checking the IIS metabase (C:\WINDOWS\system32\inetsrv\metabase.xml ) showed that he MIMEMap was set correct, i.e:

…
.docx,application/vnd.openxmlformats-officedocument.wordprocessingml.document
.pptx,application/vnd.openxmlformats-officedocument.presentationml.presentation
.xlsx,application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
…
 
However, when inspecting what came back from the server with Fiddler, it showed the following:

HTTP/1.1 200 OK
Content-Length: 12620
Content-Type: application/octet-stream

Instead of the expected:

HTTP/1.1 200 OK
Content-Length: 12620
Content-Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document

So the MIME type was correct on the server but still the wrong content type was returned.
What showed to be the case here was that the server was running Episerver and it shows that this is a known issue:

"Office 2007 files behaves as compressed folder"
https://www.epinova.no/en/blog/Lars-Oyvind-Bodahl/dates/2009/12/office-2007-files-behaves-as-compressed-folder/

the solution is to install the:

"Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint File Formats"
https://www.microsoft.com/download/en/details.aspx?displaylang=en&id=3