From Microsoft Knowledge Base Article - 326965:
When you request a file from an IIS 6.0 Web server, and the file has a file name extension that is not a defined MIME type on the Web server, you receive the following error message:
HTTP Error 404 - File or directory not found.
Earlier versions of IIS include a wildcard character MIME mapping, which permits IIS to serve any file regardless of its extension. IIS 6.0 does not include this wildcard character MIME mapping and does not serve any type of extension that is not defined at the MimeMap node in the IIS metabase.
Click here to see the original Microsoft article