Where are attachments stored when syncing?

I see that attachments are stored in $HOME/.config/joplin-desktop/resources. However, I'm not able to determine where (or if) these attachments are being synchronized.

I'm currently syncing to a local directory, which gets synced to an offsite service. I do not see attachments in that sync directory.

I want to make sure I'm not going to lose attachment data in syncs/backups.