Yeah, I like the way Thunderbird did that. You can choose any “File Link” plug-in for whatever cloud file storage site you want to use. Maybe a “WebDav File Server Instance” input field in the settings would be best, similar to how Delta Chat has a “Video Chat instance” field (except it would probably need authentication settings there, too).
Another possibility might be to use Jirafeau file sharing instances. Jérôme Jutteau / Jirafeau · GitLab That’s an open-source server project similar to how WeTransfer or SendGB works where you upload a file and it gives you a link. There’s some API info here: Jirafeau, your web file repository