One fine busy day, a fellow system administrator called for help on distributing "proxy.pac" file using Windows 2003 IIS server. She says it used to work in Windows 2000 server based web server, but not with Windows 2003. After 3 system administrators spend a little precious time, we found out this following MSKB 326965 article.
IIS 6.0 does not serve unknown MIME types
What do we know!! IIS 6 is locked down for serving unknown file types by default. Earlier versions of IIS, all MIME types (*) are allowed by default. Good for us as long as we know how to override when needed.
Keywords: IIS 6, Windows 2003, 404, file not found, cannot download file,"HTTP Error 404 – File or directory not found."
Anand, the Architect 