Several of context variables may not return a value if a custom context type is being used
${request.mainpart}
${request.parts[0]
${request.parts.0.body}
${request.parts.0.mainpart}
${request.parts[1]
${request.parts.1.body}
${request.parts.1.mainpart}
Add the cluster wide property contentType.otherTextualTypes with the value of custom context type being used.
Example: application/vnd.lotsys.motors.event.request-1+json