If you’ve started putting Office 2007 documents (in the new XML formats) onto web sites, for example SharePoint, you may have encountered an issue when downloading them again.
This is because the web server needs to set the MIME type appropriately if you want the document to open in the appropriate Office application.
In IIS 6, open the IIS Manager tool, right-click on the server entry and choose Properties. Click on the MIME types button. Then, for each entry in the list on the above blog, click New and enter the file extension (remembering to start it with a full-stop) and the content type.
When you are done, enter IISRESET at the command line to restart the IIS processes.