Magic wormhole seems to be the best tool for sending someone a file. The sender selects a file and their client contacts a relay server which gives them a channel and multi-word code which they can send as a message to their friend who needs the file. Once the friend accepts the connection the file is send peer to peer and the code (and encryption keys) are invalidated. Only one guess can be made on a relay server's channel, a failed guess aborts the connection so it's extremely unlikely an attacker could guess the code first try.

https://github.com/magic-wormhole/magic-wormhole https://www.youtube.com/watch?v=oFrTqQw0_3c&feature=youtu.be

The original implementation has stagnated somewhat. There is a new one written in go, which includes a cli and web client. This one uses webrtc which is more reliable at circumventing firewalls in a peer to peer connection.

https://github.com/saljam/webwormhole

go install webwormhole.io/cmd/ww@latest

On Debian you might need to install it the old way:

git clone https://github.com/saljam/webwormhole
cd webwormhole
go build -o ww ./cmd/ww