However, in case of webmailers both the web and the mail server are not under your control.
To suppress sending of attechments through webmailers you'd have to inspect the unencrypted datastream. Meaning you'd have to either
- block SSL encrypted sessions to the webmailer
- break the SSL encryption for connections to the webmailer
However, that could get you into all kinds of privacy-related trouble, so I'd advise against doing so. Plus, an inside attacker could still encode any given file and send it inline, so just blocking attachments won't really help you here.