Firejail: how to mount ~/Downloads above a `--private` filesystem?

In short for those who do not know firejail. Firejail is a sandboxing tool aimed both to separate stuff giving the least privileges to apps, and to provide a reasonable security layer for some apps.

It mainly works by mounting special directories to allow or forbid stuff.

What I want is to mount ~/Downloads from my real filesystem onto ~ from the firejail mount point (which in reality can be resided somewhere in ~/.cache/myApp in the real FS).

Is there any know way to do that?

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