RabbitMQ dashboard does not allow the end user to change payload encoding and always defaults to string.
For one-off manual testing, it's sometimes useful to use encoding like base64. Especially useful when the underlying message is protobuf.
You can do so with the following command:
rabbitmqadmin publish --vhost=<vhost> exchange=<exchange> routing_key=<key> properties='{"type":""}' payload=<msg> payload_encoding="base64" -u <user> -p <password>