4. generate API token
5. copy the link to your private file, and paste as plain text.
6. a file id is in the link, somewhere
7. try out the file id by visiting links like:
10. slackbot warns you, and only you, once and only once
that someone found that link. this notification may
get buried, or forgotten about. it is your only chance
to revoke the public link.
11. if you forget about that link, and it goes viral with
millions of visits, lots of luck gentlemen!