There's 2 different RFE's on this you may wish to vote for, both are currently planned for GNR (6.0):
Bug 20844 - Ability to add files from file storage to email [when composing a new mail]
Bug 27703 - Cannot add email attachments from Briefcase
From briefcase item right-click:
-Send link(s) [this is what exists today]
-Send as attachment(s)
(& the same for multi-file selections)