The Encryption Contexts feature is probably what you want:
Encryption support
I've seen customers using it with a simple 'Encrypt Attachment' checkbox on the attachment upload dialog. You could also probably customise it to always encrypt attachments.
The main downside is that the 'contexts' that allow particular users to decrypt the attachments and view the contents are specific to the instance. So you can't send the attachments out from the instance in emails (technically you can, but they'd be encrypted and so unreadable to third parties). However there are customisations that might allow you to do this if it's something you need:
https://community.servicenow.com/thread/162811
https://share.servicenow.com/app.do#/detailV2/f2d4d6e0877759008bf84b0b0e434d9a/overview
