Answer
Enable Object Connection for the DocReferenceObject service on both logical units (header and line) via the relevant entity's Object Connection setup — after adding a new object connection, refresh the Object Connection cache and have affected users log out and back in before the attach option appears. If you instead want a document attached at the header to automatically also appear on every line (or vice versa), set up an Object Connection Transformation between the two, rather than relying on users to attach it twice.