Have a central system that holds the files. It can be one of the systems running JBoss AS or a separate system. Mount the drive/directory holding the files onto each system running JBoss AS. Then the app(s) can store the uploaded files on the central system, and each app has access to the files.
Thanks, PeterJ, that's a good suggestion. But the problem I see is if I want two or more servers to hold and serve the files (for scalability and redundance).
The best idea I have until now is that the web app running in Jboss AS stores and retrieves the files from another service (running eventually on more than one server) via RMI or something like that, and expose that service using JNDI. However, it could have too much overhead (in the solution you told me, network transfers are almost the only overhead).
I'm not an expert neither in RMI nor JNDI so I'm not sure of the technnical feasibility of this... I must learn more about it yet.