There is no way to control attachments other than to turn them on or off per usergroup. I think that for big boards, especially, there should be some sort of mechanism to limit either the total number of attachments a user may post on the forum, or the amount of storage space their attachments can use.
This is necessary for us since our server is running solid state drives, and due to the expense, we do not want to run out of room on these smaller drives, nor do we want to add two more drives (one primary and one "shadow" drive as a backup) just so members can use us as a free image hosting service.
Currently, we can only control the pixel size of uploaded images (the maximum file size is the size of the file they can upload, and it is downsized automatically to the maximum pixel size), and turn on/off the ability of members to upload attachments.
Because of the quotas, members would need a way to manage their own attachments--in other words, as they run out of space, they need the ability to be able to delete their own files. Ideally, a page of image thumbnails (or placeholder icons, such as with PDF files which don't have thumbnail views) with checkboxes, perhaps with short links to the post(s) they are used in, would allow members to easily delete them. There should also be an option for members to purge attachments prior to a given date. (I've found on our big board that very few older threads with attachments are ever viewed and, as such, the attachments can be deleted without anyone missing anything.)
Quite frankly, I'd rather see them host the images elsewhere and link to them in their posts, and we may have to go that route for the time being, until some sort of quota system could be put into place. We are also contemplating adding a subscription (of a minimal amount) per year for members who want to upload attachments, and that funding would go toward another pair of drives. But still...if you give some members an inch, they'll take a mile (as the saying goes).