There is actually no real filesystem in the Blackberry, however, there are methods of storing data, but for a real-looking filesystem, you will have to write your own filesystem emulator that piggybacks on the existing methods of data storage. Writing such a filesystem emulator is a lot of work. It's a lot easier to license eFile if a filesystem is critically important to your application.
I dug up some hyperlinks that might be useful. Let me know if they help!
For generic midlet development:
SUN J2ME Developers
For Blackberry specific development:
Blackberry Developers
For Info on Blackberry extensions to Sun Java J2ME MIDP/1.0
Introduction to Java and J2ME on Blackberry