[JENKINS-73124] MIME_TYPES should not be manipulated

[alecharp]

This comes from jenkinsci/jenkins-test-harness#764 when the MIME_TYPES manipulation is deprecated and should be removed in order to complete the migration to Jetty 12.

It follow up the example of jenkinsci/kubernetes-plugin#1547.

Testing done

I ran mvn verify and no test failures were reported.

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