Symfony version(s) affected: <=4.3

Description

All messages are published to rabbitmq with content_type as plain/text, even though they're serialised as json. It makes it harder to consume messages with other technologies (like spring), which recognise the header to decide how to decode the message.

How to reproduce

Configure the messenger to use the symfony_serializer instead of the php one:

framework:
    messenger:
        transports:
             amqp:
                dsn: 'amqp://admin:admin@localhost:5672/%2f/foo'
                serializer: 'messenger.transport.symfony_serializer'
        routing:
             'App\Message\Foo': amqp

Publish a message and inspect content_type in rabbitmq.

Possible Solution

During serialisation create an AmqpStamp instance with the content_type attribute set based on the serialisation format.

The drawback would be that only the last AmqpStamp is currently read, so any AmqpAdmin stamps add by user land code would override it.