Because there isn't a true file browser (like there is on your BlackBerry Phone) yet, you can't access your files unless you use your computer.
An exception to this is viewing video files in the video app, pictures in picture app, excel files in excel app, etc....but you can't delete files through those apps. To truely play around with the file system, you need to acces the PlayBook through your computer (either hooked up through USB or as a WiFi network drive).