I recently ran into an interesting issue where I had a .xls file linked in one of my web pages, and changed the content to a .xlsx file ... updated the link and it quit working.  Every time I clicked on it, I was sent to a 404 error ("Page Cannot Be Found").  I double checked the path in the link, that the file actually existed, that it had the correct permissions, etc. and everything seemed correct.

Turns out you have to manually add the MIME Types for the new Office 2007 file extensions (e.g. docx, xlsx) in order for IIS 6 to render them.  I checked IIS 7 and it apparently doesn't have this same problem.  Here is how you add the new types:

1. Go into IIS Manager, right-click on the Web Sites folder, and select Properties
   
   
2. Click on the HTTP Headers tab and then click on the MIME Types... button
   
   
3. Add each of the MIME Types contained in the table below
   

New Office 2007 MIME Types