Apache Config Mime Types: xlsx files are being interpreted as zip files on download, why?

Question

Background: For some reason, whenever a user tries to open an xslx (excel 2007) file from our intranet using MSIE, the file download dialog interprets it as a "zip" file.

Admittedly, xslx files really are zip files, but we don't want that behavior. Just open in Excel, please.

Question:

Firefox, OTOH, opens the files normally. Is it possible that the fault is my apache configs? or is this a client-browser-only issue?

Answer 1

The reason is IE's strange MIME sniffing behaviour: http://msdn.microsoft.com/en-us/library/ms775147.aspx. It basically tries to determine to the file type of a file by looking into its contents, somehow like Linux's file tool.