I come back to this discussion because, when I'm reading the doc I find that a detail is not very homogeneous.
in the doc: https://jolokia.org/reference/html/agents.html Section: JEE Agent (WAR)
a parameter is present to set the default mimeType
mimeType MIME to use for the JSON responses Default: text / plain
Allowing in JEE to configure application/json as default
As for the agent: OSGi Agents
this configuration is impossible.
to make things uniform I think this parameter should be present for all agents.