I am honestly also chocked by the absence of this feature! Your servers receive the emails (BCC and Forward) with the attachments, so why exclude the attachments when saving the emails in your system?
Our previous CRM system, Highrise, had this feature of course and I did not expect at all that I had to check that this feature existed in Hubspot CRM as well before we made our choice to switch system…
I really, really hope this feature will be added soon!