You can also do this through applets using an RSA certificate (either official or self-signed). After a user has accepted you as a trusted site, you will have access to their file directory. We are using this to allow users on our intranet to download binary files, since we have 60,000+ files. Self-signed works well for intranet.
Some useful websites for self-signed certificates are: http://www.suitable.com/docs/signing.html http://java.sun.com/products/plugin/1.3/docs/nsobjsigning.html http://forum.java.sun.com/thread.jsp?forum=63&thread=174214