https://pairdrop.net is FOSS, cross platform, realtime, peer-to-peer, and only needs a browser. You can host your own version if you prefer. In contrast, Firefox Send (also FOSS) was ‘asynchronous’ (you could upload, and then email a link), but it was shut down due to abuse. https://github.com/timvisee/send is a fork of the archived github project that you can self host with many improvements, notably authentication, so only yourself and trusted users can upload. (edit: wrong link for ff send)
Delta Chat just a clever chat interface over SMTP/IMAP, and thusly every user can use their own mail server for sending + receiving messages. Any encryption is between the end devices, and to cooperate, they’d have to produce a compromised build that leaks those keys.