There’s a bug in the current Forum software version that makes certain file uploads not work:
There seems to be a quick fix to this problem, but I’m afraid it might cause problems when updating to the next software version. I might avoid that issue, but that would mean more work for me.
If someone needs to upload something soon, please reply and I will try to apply the bug fix. Otherwise I’ll just wait until the next update in which the bug should be completely fixed.
I’ve just updated the forum software to the latest version and the bug is fixed – uploads / downloads should work properly now (again) 
No need to apply that fix/hack.
There still seems to be a bug when it comes to file uploads in private messages. The uploads work, but the link in the message is broken. If you want to be sure attachments work, please rely on the old-school email system 
Argh, it seems it’s really a commonly occurring bug now. I’ll try to apply the following fix tomorrow morning. Please be warned that this may cause the forum to be unavailable for a while.
I’ve just had to realize that the typical Docker install for Discourse makes it relatively easy to install Discourse, but actually changing anything becomes a real challenge. I’m afraid that I could mess things up badly if I try to hack the forum software somewhere inside the Docker installation files.
This means, that I admit defeat for now. Attachments have a high chance of not working until the next version of Discourse comes out. If you want to share files, then please use external file storage services and simply share links to your external files from Discourse.