Are you aware of #Lufi?
It allows to store/download files that were previously encrypted from the client side. Yes, data will never leave your computer without being encrypted :)
If you are interested, it's here: https://framagit.org/fiat-tux/hat-softwares/lufi/blob/master/README.md
And you can also test it: https://demo.lufi.io/
Enjoy this nice work from @framasky