Requests for static files return 404 error (IIS 6.0)
For requests to static content, this version of IIS serves requests for files with known file name extensions only, a feature called Known Extensions. If a request is made for a resource whose file name extension is not mapped to a known extension in the MimeMap Metabase Property, IIS denies the request and logs a 404 error with the substatus code of 3 (404.3) in the W3C Extended log files (by default). To prevent disclosure of configuration information, IIS is configured to return the generic 404 custom error page by default on this denial. To resolve this error, use IIS manager to add the file extension to the Multipurpose Internet Mail Exchange (MIME) map on a specific directory or Web site.
If adding the specific file extension to the MIME map does not remedy the problem, turn off the Known Extensions feature on a specific directory or a Web site. To turn off the Known Extensions feature and thereby allow IIS to serve files with any extension for a specific site or directory, add the *,application/octet-stream value to the list of MIME maps (enter the asterisk in the Extension text box and application/octet-stream in the MIME Type text box on the MIME Type dialog box). For more information on how to add this value to the MIME map, see Working with MIME Types.
If you are still experiencing problems, you can turn off the Known Extensions feature on the Web server and allow IIS to serve files with any extension for all sites and directories. Turn off the Known Extensions feature by adding the *,application/octet-stream to the global MIME map. You must wait until the worker process has recycled or restart the World Wide Web Publishing Service (WWW service) before changes take effect.
Turning off Known Extensions on the Web server means IIS will serve files with any extension. Malicious users could potentially exploit this configuration to download sensitive information from your Web server. An IIS server that serves files with any extension is not secure.
Tools like UrlScan can be configured to block the processing of certain file name extensions. Check your UrlScan settings.
For more information on adding or editing the MIME map, see Working with MIME Types.