IIS 6.0 does not serve files with .docx extension

Today, our support team got the ticket from the customer that one of our online applications does not seem to recognize the .docx extension. The client was able to upload files to the system, but was not capable to open them from web application. I tried by myself and when I request a file with .docx extension from an application served from IIS 6.0, I receive the following error message "HTTP Error 404 - File or directory not found", even though the file is there.

The solution came from Microsoft's article found with Google search IIS 6.0 does not serve unknown MIME types. Based on Microsoft's approach I compiled together as few steps to resolve the issue as fast as possible:

1. Open the IIS Microsoft Management Console (MMC), right-click computer name, and then click Properties.
2. Click MIME Types.
3. Click New.
4. In the Extension box, type .docx
5. In the MIME Type box, type application/vnd.openxmlformats-officedocument.wordprocessingml.document
6. Apply the new settings
7. Recycle application pool.