I got HTTP Error 404 - File Not Found when browsing files without extension, the same file could be seen properly with txt or html extension.
For requests to static content, 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.
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.
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).