Since 2.x the official supported JUnit report schemas are the most used in Java world (ant and maven surefire).
The breaking changes was introduce (since 2.x using semantic versioning), a warning is shown in the Jenkins plugin page and all details provided in the Wiki page
To provide supports for the who use tools which declare to produce a JUnit report that is not strict to any official schema, the procedure is configure the xUnit plugin select the "Custom Tool" as test type and provide the path to a custom XSL (it will be provided by this plugin as attachment and documented in Wiki page) in the "custom stylesheet" textbox. The path of the XSL file that can be absolute on the server (jenkins master) or relative to the workspace or an URL (see the wiki page for more details).
The XSL is free and could be customised to fit every extra needs.