I used to have a downloads system which let you add a download, but without uploading it to your server, but simply from a link.
I think i know the answer, but is that possible with Xenforo in any way? To have an attachment working in the same way? Just that I need to have an attachment that gets updated on an external server very frequently, and is also tracked, and also part of the forum. It would be impractical to keep uploading it.