... save game. What you describe is a save game. Play some twine games. They usually have that feature. There is save to disk and load to disk. So it is possible.
But if you need the user to generate the file, download it, and make them send it to you with email, you might do that directly in the webgame. I mean, there are also such things as global high scores and other things were information flows from the user over the game to a server and even back into other people's games.