Borislav is right, although it seems to me that overriding the model's save method would be more appropriate in Your case.
On the other hand, You really should think twice before doing so. There's actually a good reason for django orm not to delete the actual file after the corresponding FileField had been changed or deleted. Your models' edition/deletion is usually handled in db transactions that are rolled back when something goes wrong and unfortunately there's no way to undo a file's deletion...
Perhaps it's better to periodically run a script that deletes orphaned files.