Currently there is no way in confluence to purge old versions of attachments.

1. It should be possible to erase old versions of attachments and not only the whole attachment including its history. This is to reserve space
2. It should be possible to ask for a report of attachments, sorted by size with their versions and ask to delete versions older than a specific time.
3. It should be possible to ask, per space or even per wiki, to erase old versions of attachments old than spefic time.