Validates that this envelope is a valid attachment, in the reference's
order (validate_attachment): the payload is read (NotWrapped when the
object is not wrapped), then the vendor and the conformsTo, then the
attachment is rebuilt from them and must be equivalent to this envelope
(which also checks that the predicate is 'attachment').
EnvelopeError with code InvalidAttachment when the envelope is
not an assertion or does not rebuild; NotWrapped,
NonexistentPredicate, AmbiguousPredicate or Cbor from the parts.
Validates that this envelope is a valid attachment, in the reference's order (
validate_attachment): the payload is read (NotWrappedwhen the object is not wrapped), then the vendor and the conformsTo, then the attachment is rebuilt from them and must be equivalent to this envelope (which also checks that the predicate is'attachment').