With the release of Microsoft Office 2007 came the release of the new Microsoft Word extension ".docx"”. This new format has come with some expense to end users. Older versions of Microsoft Word can't open the new file format without a compatibility update from Microsoft. While the compatibility update addresses one issue for end users it doesn't address another one related to the web and ISS web servers.
When trying to create a link on a web page to a file ending in the .docx extension a user will receive a 404 – File not found server error.
If this is occurring, chances are that your IIS server hasn't been configured to allow the delivery of .docx files. To resolve the problem you will need to add a mime format for this extension.
In IIS, right click on the site that is trying to deliver the .docx files and click properties.
Your .docx files should now be registered with the IIS server and will no longer return a 404 – File not found server error.
Web Department Manager