If anyone wanted to send a file they could write it to a CD, DVD, or USB stick and post it. They can leave it at a dead letter drop, under a bench in the park.

If anyone wanted to send a file they could write it to a CD, DVD, or USB stick and post it. They can leave it at a dead letter drop, under a bench in the park. One can even print it on paper as bar codes or similar and send it in the post so that simple external inspection would not reveal that it was not just an ordinary letter.