Why are you showing me the application pool, look in the selfupdate within the default site.
You also have two other websites defined which might be possible that one of them is bound to the IP that fileserver has while the default site is using the all unassigned IPs port 80.
On the 'fileserver' try
http://127.0.0.1/selfupdate/wident.cab. do you get an error or do you get a prompt to download/save the file?
You could enable directory browsing on selfupdate to avoid the download prompt.
if either of the two websites you have defined is bound to the fileserver LAN IP on port 80, that would explain why WSUS clients get the 404 error.
