Worked it out.
In the admin panel:
then tick the 'upload files' box for registered users.
Thanks for NodeBB It works fine here with Firefox. However, I tried to use it with Epiphany/Web (the Gnome browser) and the admin panel doesn't work, i.e. when I click on some menu entry in the Admin panel, nothing happens (a short flash in the page indicates that something actually happens, but the display is not refreshed then).
I don't know whether this is an Epiphany or a NodeBB issue and it's not imporant (because this browser is used rarely and has some other issues), but I thought it could be interesting for you and maybe it's a problem that could occur with other browsers, too.
I think you're right when you say probably not many people use it. Best bet - submit an issue to them and ask why it doesn't work, if they can give us a clue as to what we should do I'll be more than happy to give it a shot.
Just FYI, I don't do anything magically special on the new ACP. Went for a very clean and basic look so its unfortunate it doesn't work on this browser.