I'm going to fix this. But just out of curiosity, why do you need this?

This should be fixed in gradle-jpi-plugin, so that all plugins which are built with Gradle will get the fix.

https://github.com/jenkinsci/gradle-jpi-plugin/pull/66

Code changed in jenkins
User: Daniel Spilker
Path:
CHANGELOG.md
src/main/groovy/org/jenkinsci/gradle/plugins/jpi/JpiManifest.groovy
src/test/resources/org/jenkinsci/gradle/plugins/jpi/basics.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/compatible-since-version.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/complex-dependencies.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/dependencies.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/dependency.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/hpl-basics.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/mask-classes.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/optional-dependencies.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/optional-dependency.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-class.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-developer.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-developers.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-first-class-loader.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/sandbox-status.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/support-dynamic-loading-maybe.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/support-dynamic-loading-no.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/support-dynamic-loading-yes.mf
http://jenkins-ci.org/commit/gradle-jpi-plugin/cb38083d98eb58dc9838ed6993f0657628382cf6
Log:
added Extension-Name entry to MANIFEST.MF

JENKINS-31542

Code changed in jenkins
User: Daniel Spilker
Path:
CHANGELOG.md
src/main/groovy/org/jenkinsci/gradle/plugins/jpi/JpiManifest.groovy
src/test/resources/org/jenkinsci/gradle/plugins/jpi/basics.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/compatible-since-version.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/complex-dependencies.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/dependencies.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/dependency.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/hpl-basics.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/mask-classes.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/optional-dependencies.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/optional-dependency.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-class.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-developer.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-developers.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/plugin-first-class-loader.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/sandbox-status.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/support-dynamic-loading-maybe.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/support-dynamic-loading-no.mf
src/test/resources/org/jenkinsci/gradle/plugins/jpi/support-dynamic-loading-yes.mf
http://jenkins-ci.org/commit/gradle-jpi-plugin/a1579a9974feae327595f48b5295661565f6830e
Log:
Merge pull request #66 from daspilker/JENKINS-31542

JENKINS-31542 added Extension-Name entry to MANIFEST.MF

Compare: https://github.com/jenkinsci/gradle-jpi-plugin/compare/ab6ad8ca29a6...a1579a9974fe

Has been fixed in gradle-jpi-plugin 0.14.1.

Pull request for job-dsl-plugin to update to gradle-jpi-plugin 0.14.1: https://github.com/jenkinsci/job-dsl-plugin/pull/676

Code changed in jenkins
User: Daniel Spilker
Path:
job-dsl-plugin/build.gradle
http://jenkins-ci.org/commit/job-dsl-plugin/a72a93a368b8743a56fa60c4bad18ad92df591a9
Log:
Merge pull request #676 from daspilker/JENKINS-31542

updated to gradle-jpi-plugin 0.14.1

Compare: https://github.com/jenkinsci/job-dsl-plugin/compare/1b5e7811b848...a72a93a368b8

Will be fixed in job-dsl-plugin 1.41.