Webservers can be configured to serve pac files directly pointing to
a server name and port (without specifying a pac file at the end of the URL)

e.g.

Following works through the plugin

http://<server-name>:<port>/file.pac

but if webserver is configured so that file.pac is obtained by just pointing
to

http://<server-name>:<port>

then java plugin is not able to handle such situations.