check whether basic auth works for private files

Problem/Motivation

> And the URL for the URI private://my/file.jpg will not authenticate you with basic_auth if I am not mistaken. Or if Ok with basic_auth, we need a dedicated method for other authentication plugins.

Either:

- it doesn't, and we should document that
- it does, and we should change the code

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes