On many messaging platforms, the server provider is also responsible for providing a way for clients to upload attachments. This removes the need for clients to maintain/pick their own upload service.

@progval has written a proposal: https://github.com/progval/ircv3-specifications/blob/filehost/extensions/filehost.md

Prior art:

• https://xmpp.org/extensions/xep-0363.html
• https://spec.matrix.org/v1.3/client-server-api/#content-repository