Looks good.  The banner might also show how to contact the group's moderators, since the person who sees this has little or no control over freeing up space.

Speaking of which, does it also send a message to the moderators to let them know that an upload attempt failed and they must intervene?

Even better, maybe, it could have a button to bring the member directly to editing a message addressed to groupname+owner@groups.io, partially filled-in about the lack of space.  OK, maybe too much.

Andy