It looks to me as though it's not possible, within AquaMail, to designate a folder - on any cloud storage location - for attachments. The best I've been able to figure is to store attachments locally, then use a third party application to sync the contents of the local folder, to Google Drive (or OneNote or whatever). There are several apps that do this. I'm using Autosync Google Drive, from MetaCtrl.
And then maybe I'll add "saving attachments to a cloud folder" as one of my AquaMail feature requests. . .
