Yes, exactly — sites which host media uploaded by users are all right so long as that's not the primary purpose of the site. (So, for instance, allowing users to upload MediaWiki images or forum avatars is perfectly OK, but running a PhotoBucket clone isn't.) In this case, accepting payments to get access to more storage makes it clear that you're basically just trying to resell storage/bandwidth.
If you're willing to host the downloads externally (through a service like Amazon S3, for instance), though, that totally changes things. If you're fine with the idea of paying extra for the storage/bandwidth your downloads use, that might be the way to go.