FormBody was checking for the existence of an element within the envelope, then altering the content disposition, rather than just implementing a setter on the Envelope. This was likely to avoid changing the interface for a node (why was behaviour pushed out to FormBody?).
[x] Update NodeInterface (mutable method added on Node)
[x] Relocate logic
[x] Bring all descendants of NodeInterface into line with Interface
FormBody was checking for the existence of an element within the envelope, then altering the content disposition, rather than just implementing a setter on the Envelope. This was likely to avoid changing the interface for a node (why was behaviour pushed out to FormBody?).
NodeInterface(mutable method added on Node)NodeInterfaceinto line with Interface