An option to save an email attachment from protonmail to protondrive without download
Conspire with the Universe shared this idea
And a possibility to attach directly files in proton drive as email attachment as well!
Joshua Hardwick commented
Perhaps this comes after an in-browser viewer becomes available. Currently the file needs to be downloaded anyway to be viewed. But there may be times where a file gets emailed to a person simply for them to store in Drive. I can see that often being the case since Drive cannot be shared with others for the purpose of the other person uploading new files.