I just pushed the new Files section to the live site. It should be pretty self explanatory. It has the same permissions types as the Calendar. Moderators have a new notification check box to indicate that they are to receive notifications when the file space is full. At the moment, no notifications will be sent if that happens; I'm still working on that.
The disk space used is calculated over the group and any sub groups, and includes all space taken up by attachments in the archives and files uploaded to the Files section. I have not decided how to handle the situation where a group is over its' disk space allocation and someone sends a message with an attachment, but I'm leaning towards bouncing the message back to the sender with an error message.
All operations in the Files section are logged in the Activity log.
I'll announce this on updates next week. Please let me know if you have any suggestions for improvements or bug reports.