Management command filer_check does not find orphaned files in the private storage

[MarcoSteinacher]

The command ./manage.py filer_check --orphans (and --delete-orphans) does not scan for orphaned files in the private storage («Secure Downloads» / smedia). Only the media directory is scanned.

I fixed the problem for my use case here, but since I'm not familiar with all the internals of django-filer, I'm not sure if that is done in a proper way.