Currently, the new site backup experience within admin.atlassian.com allows admin users to backup directly to an S3 bucket. However, attachments are still handled in the older method in which only the reference to it is included:

https://support.atlassian.com/organization-administration/docs/overview-of-atlassian-backups/#Data-retention

You can’t download files and attachments because we store only references to the files and not the actual files. This reduces the time to run backups.

Giving admins the option to also include the actual file itself to be included in the full backup would be useful to those users advanced enough to access S3 and manipulate the data as need be, and the default experience can be left as is for those who have no need to access the backups directly.