the actual attachment is here;topic=1009.0;attach=1107

1. There is no way to separate the filename and the file path, because the above URL did not contain file name.
You can use archive="the actual attachment is here;topic=1009.0;attach=1107" and without codebase in the applet tag to solve the problem.
However, the above URL did not link to the jar file. Because it will be re-direct to login page.
May be you will not see the login page , because browser did the login trick for you.
However, it you logout your forum and try to aceess the above URL again, it will be re-direct to login page.

The same things happened when java/other users try to access the above URL.
That is the reason why java was not able to find the required jar file with the above URL?java should not be able to acces your server because java program can not login)