OnionShare

OnionShare is an open source tool that allows you to securely and anonymously share files of any size, host websites, and chat with friends using the Tor network. There's no need for middlemen that could very well violate the privacy and security of the things you share online. With OnionShare, you can share files directly with just an address in Tor Browser.

OnionShare works because it is accessible as a Tor Onion Service. All you need to do is open it and drag and drop the files you want to share into it, and start sharing. It will then generate an unguessable address which the recipient can open in the Tor Browser to download the files; they won't even need their own OnionShare.