You can send binary WebSocket messages from AutobahnPython and receive those on browser-side with pure WebSocket (you don't need AutobahnJS for that).
AutobahnJS is a WAMP implementation: RPC and PubSub over WebSocket. WAMPv1 uses text (JSON) messages for serialization and does not support (unencoded) binary payloads.
There are considerations for supporting binary payloads in WAMPv2 (https://github.com/tavendo/wamp/issues/4)