This morning I got a call from an Administrative Assistant about a new Office 2010 document not working on our intranet web site. It was giving her the 404 File not Found Error. After confirming the URL of the file was correct, it dawned on me that IIS has locked down the MIME types. We recently upgraded our computers from Office 2000 to Office 2010, so that means new file extensions are in effect: docx, xlsx, pptx, etc.
Here's how to fix the problem in Internet Information Services 6.0 (IIS):
1. Open your IIS Manager.
2. Right click on Web Sites.
3. Choose Properties