Very good questions.
My “real” attachments I keep in Google Drive. There is no way to easily export attachments at the moment so this way I have everything at hand. My workflow in that regard is described here
I might just set my DL permissions to be viewed by all. This way I can easily share the attachments, my primary reason to ask for File Level permissions.
In the end, layering the file manager on top of Dropbox and/or Google Drive might be the better way to go. The whole file sharing, uploading, and management may add just too much of an extra development load.
Advantage of doing it in-house might be that backups for Pro could have the attachments included, in certain scenarios.