ActiveStorage specs for versions 6.1 and up don't reflect several methods that are delegated from the Attached::* objects to the Attachment object, and from Attachment to the Blob model. For example, url and filename are both available on Attached::One through Blob, but either have no signature (filename) or an inaccurate signature (filename). In part this is due to incomplete specs, and in part it is due to symlink delegation to earlier versions.
I will try to supply more detail when off work.
Relevant links: