For new Yammer groups integrated with Office 365 group there will be two places for storing Files and Notes. One is Yammer container (as in current Yammer groups) and second is Office 365 Groups Files and Notes stored on SharePoint.
When can we expect to have only one place to store files and notes? Having two places can be very confusing for users.
We recommend that you move towards using SharePoint and OneNote if you
have an Office 365 connected group. Since we are still rolling out
connected groups over waves, it will be some time before we move away
from Yammer Files and Notes, but they do help with quick, unstructured
ways to share documents and notes. For example, sharing a picture
quickly stores it in the Yammer file tab.