ResourceToolActionPipe provides a conduit through which ResourcesAction and an
unknown helper may communicate about the execution of ResourceToolActions in which
the registered action specifies that some part of the action is handled by a helper.
ResourceToolActionPipe has a set of methods through which ResourcesAction can pass
information to a helper about the current state of the entity (or entities?) involved
in an action ("setContent", "getContent", "setMimeType", "getMimeType", etc).