OnionShare is an open source tool that lets you securely and anonymously share a file of any size.
It works by starting a web server, making it accessible as a Tor hidden service, and generating an unguessable URL to access and download the files. It doesn't require setting up a server on the internet somewhere or using a third party filesharing service. You host the file on your own computer and use a Tor hidden service to make it temporarily accessible over the internet. The other user just needs to use Tor Browser to download the file from you. Designed to work in Tails.