This has been on my mind, lately.
Seems to me that a prerequisite for the pasting-in function is a media library management system so when someone pasted in the wrong thing they will have a way to delete it later. How does Lemmy handle that?
A media library could hold audio and videos too, for posting those...
But we don't really want to become imgur, the storage would be immense. So really we need a way for people to BYO storage - provide the login details to their ftp site, Google drive, S3 etc and then use that as the storage provider for that user... The media library would be written in a way that it can be extended with different storage providers, perhaps defined using plugins so any instance can add their own weird custom storage solution...
It's a lot. The vision needs to be pared back to something achievable yet still useful.