Interesting, it seems to work very similarly to Firefox Send.

For communications in the other direction, such as when you want to make sure the other person sends you the content securely, there is https://github.com/whitesmith/hawkpost (disclaimer, this was an hackathon project I participated some years ago). Hawkpost doesn't even store the encrypted content.

I wonder if there is a solution that would correctly deal with both situations (never requiring the other end to sign up or know anything about encryption).

Thanks for the link to Hawkpost, it looks really cool!

you might also be interested in https://github.com/magic-wormhole/magic-wormhole

one of my favorite tools, i just wish there was a good GUI for it.

There's also a nice go port (single binary) of it: https://github.com/schollz/croc