Having been confounded by this before I new exactly where to look when I couldn’t get a PowerPoint 2007 file to download via IIS. (Properties/Http Headers/MIME Types… to be precise).
What I didn’t know is what the MIME type of a .pptx file should be which led me to this list of files and their MIME types. UPDATED LINK: list of mime types Excellent, comprehensive and up to date list beautifully rendered with some nice Comic Sans and a matrixesque page background 
Thanks for this tip! It was just what I needed.
the link to file and their MIME types is 404, can you update? I’m looking for the MIME type for .docx
I’ve updated the link but the list I’ve added doesn’t contain the Office 2007 formats.
There is a list here of those:
http://www.bram.us/2007/05/25/office-2007-mime-types-for-iis/
Graham