Considering the fact that Storage is usually limited it would be nice to see all Attachements one owns, or received, in one place. Perhaps even implement a reminder for rarely to never accessed files so the User can download a copy and simply have it removed to free up space. After all, not everyone wishes to continuously flood a system and have it ever expanding.
That being said, I understand users who wish to never worry about this, thus it should be optional or not an intrusive system. An occasional, monthly to bimonthly reminder would serve plenty on cutting down on needed server capacity I think.
Reducing some power consumption in this manner I think would be a nice optional thing to have and also raise awareness on topics regarding green IT. After all, the same happens for sitting in a call all alone, to preserve bandwidth. Though those costs are on a different scale.