Looking at the documentation docs & attachments are stored in SP
Your Jive groups will store attachments and images in SharePoint
But can this be mandated across a whole site, or just to groups connected to SP?
This can be mandated across all Groups, Spaces and Projects by configuring SharePoint as the default storage provider for your community under the "Storage Management" tab. SharePoint is one of a number External Storage Providers support by Jive and all of them can replace Jive as the default binary document storage repository. The screen shot below illustrates where the default storage manager for Jive can be changed.