Since Jetty 12 doesn't support mutable MIME types, I have deprecated this field in https://github.com/jenkinsci/jenkins-test-harness/pull/764. The field should just be deleted, but before we can delete it, we need to remove the sole remaining usage in https://github.com/jenkinsci/kubernetes-pipeline-plugin/blob/master/core/src/test/java/org/jvnet/hudson/test/JenkinsRuleNonLocalhost.java#L71 as in https://github.com/jenkinsci/kubernetes-plugin/pull/1547. (I am also aware of a proprietary usage in LowThreadsJenkinsRule in cloudbees-support-plugin.) Once that is done, the field should be deleted from jenkins-test-harness, and the temporary changes made to it in the prototype branch of jenkins-test-harness should be undone.