Disclaimer: I wrote this article and made this website.
There was some talk of this issue in the recent fediverse inefficiencies thread. I’m hopeful that in the future we’ll have a decentralized solution for file hosting but for now I deeply believe that users should pay for their own file hosting.
Stuff that isn’t accessed eventually gets deleted. If the Lemmy instances (which are clearnet, of course) delete the references to it, it would go away.