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 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. You can add or edit the Multipurpose Internet Mail Exchange (MIME) map using IIS Manager. To turn off the Known Extensions feature and allow IIS to serve files with any extension, add the *,application/octet-stream value to the list of MIME maps. If you update 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. If you update an individual Web site MIME map, the change is reflected instantly.
For more information on adding or editing the MIME map, see Working with MIME Types in the Help that comes with IIS Manager.
Tools like URLScan can be configured to block processing of certain file name extensions.
Check your URLScan settings.