Found in https://github.com/jenkinsci/copyartifact-plugin/pull/147.
Class attributes of layout:icon custom tag are used to specify icons, and extra developer-defined class results to fail to find icon.

It blocks plugins migrating img to layout:icon, which triggered by https://github.com/jenkinsci/jenkins/pull/5778 .

TODO:

• Test with the latest core -> Verified reproduction with Jenkins 2.343
• Resolve the issue with:
  • A: ignore classes without icon- prefixes
  • B: add extraClass attribute
  • C: update document, and allow developers to use src if they want to specify extra class.
    • Actually, documentation of layout:icon looks buggy as it states that src is ignored if class is specified (but actually, src is checked first): https://github.com/jenkinsci/jenkins/blob/jenkins-2.343/core/src/main/resources/lib/layout/icon.jelly