You could store the files in a directory external to the web app (and to the JBoss installation entirely).
You'll then need to configure the web-app with the path to the external directory. You could define a initialization parameter in your web.xml to specify this.
Unfortunately this would make your WAR file server-specific. As an alternative you could provide functionality in your web app to capture and store this path (e.g. a configuration screen), and make it a required step after installation to set this up.
You could also bind a value into the server's JNDI tree and configure access to that JNDI entry in web.xml and jboss-web.xml. We chose to do this; we bound a Properties object to allow us to add multiple server-installtion specific paths that are accessed by a web-app.