i kindly but strongly disagree. DC’s great fundamental feature is that its transport layer is Email. To weave an other transport system into it is against its base ideas, IMO.
which cloud storage? people have widely different preferences on it.
how would it integrate smoothly to other email softwares?
how are recipients authenticated at that particular cloud provider?
(there is no cloud, just someone else’s computer.)
how non-DC recipients keep the attachments as easily as real email attachments?
how long keep the file in the “cloud”; what if the recipient does not open the link immediately? (remember, email is not meant to be instant)
with detached attachments (see the oxymoron) many DC killing feature get impacted.
as I expressed in an other thread (Smarter message size limit), an SMTP protocol extension would be the most elegant to detect max mesage size. And to “push everything through”, send huge files in chunks automatically (DC should split and concat them without user intervention).
I agree not to force users to download large messages unconditionally.
IMAPv4 supports downloading only the headers, or the message partially IIRC; so downloading whole huge attachments unsolicitedly has not to be an issue – it can be implemented in DC without ecosystem change.