You both open web.keibidrop.com, connect with a code once, and drop the file onto the page. The other person presses Save. There's nothing to install, no account and no upload to a cloud.
The two tabs talk through our bridge, which passes along encrypted pieces it can't read, and the encryption is quantum-safe. It works on a work laptop where you can't install apps, or on a computer that isn't yours.

Full version, with the limits and the method: Send a file from one browser to another, nothing to install

Source, AGPL-3.0: github.com/KeibiSoft/KeibiDrop