Keeping versions of attachments.
You will have to use some form of Document Management System behind the scenes for this scenario. It's the only way I've found that works...
There are several connectors for helping in this regard but they are limited to basic work.
From experience, if using Windows do not try to install Alfresco 4.2.f - it doesn't work. And Alfresco 5.0 does not yet have Connectors in Bonita Studio. I've used OpenKM to basic but good effect.