springintegration http inbound gateway: access full Message after conversion

Question

i'm using spring integration, i've defined an integrationflow that starts with an http request using Http.inboundGateway:

IntegrationFlows.from(Http.inboundGateway("/test")
            .replyChannel("http-response")
            .requestMapping(m -> m.methods(HttpMethod.POST))
            .mappedRequestHeaders(sharedsecretName)
            .requestPayloadType(Map.class)
            )
            .log(this.getClass().getPackageName())
            .channel(MessageChannels.publishSubscribe(SUBSCRIBABLE_CHANNEL).get())
            .get();

in subsequent channels the message is sent after the conversion according to

.requestPayloadType(Map.class)

Is there any way to access the full Message (i need to check the mapped header) instead of the converted one?