Please enable mailing attachments by Special:EmailUser for at least usergroup sysops/administrators. It's very useful for sharing research material.
Is there reason you'd choose to use Special:EmailUser over a standard e-mail client? Presumably wiki administrators could be given access to users' e-mail addresses. That would probably be less dangerous and cumbersome than adding attachment capability to Special:EmailUser.
The easier way must not exclude other ways w/o at least one trial.
Must? I mean, watch us do just that :P
To be more serious: If you had a compelling reason for how this would benefit wikimedia, we would look into solutions to mitigate the risks. The rationales provided so far arent, so we probably wont.