When adding an attachment in Fossil, if the attachment is syntactically similar to a structural artifact (such as a manifest), then the attachment is compressed prior to being hashed and stored, thus making it very dissimilar to a manifest and incapable of being confused with a manifest. Hence, it is not possible for someone to add a new manifest as an attachment and have that confuse the system. Furthermore, there is an audit trail so that should an attacker discover and exploit some bug in the previous mechanism and manage to get a manifest inserted via an attachment, then the rogue manifest can be easily identified and "shunned".
Users with commit privileges are granted more trust and do have the ability to forge manifests. But as before, there is an audit trail and rogue manifests (and the users that insert them) can be detected and dealt with after the fact.
Structural artifacts have a very specific and pedantic format. You can forge a structural artifact, but you will never generate one by accident during normal software development activities.
