Tails includes OnionShare, a tool for anonymous filesharing. It allows you to share files directly from your running Tails. To do so, it creates a Hidden Service (a website whose address ends with .onion) within the Tor network. Any Tor user who you give this address to can download the files you decided to share.
To share files with OnionShare:
Start the file browser.
Right click on a file or folder in the file browser and choose Share with OnionShare.
If you want to add more files, drag and drop them into the window of OnionShare.
If you want to allow multiple downloads, uncheck the Stop sharing automatically check box. Otherwise OnionShare will stop sharing files after they have been downloaded once.
Click on Start Sharing. When the files are available, an address similar to http://bwwijokny5qplq5q.onion/assam-cover will be shown.
Now you can pass the address to anyone else, e.g. via e-mail. OnionShare will inform you when the shared files are being accessed.
Once you close the OnionShare application or shut down Tails, the files are not shared anymore.