Dave Hill and I have been using the File Manager code from ArsDigita on various projects. I packaged the code we use with OpenACS 3.2.x and you can download it
here.
Our bug fixes have also been folded into the file-manager package for ACS4 classic, which is available from aD's download section.
It will be extremely easy to get this package working with OpenACS4, as it doesn't ever touch the database.
