It seems that the maven (in a maven project) is not picking up .mvn/extensions.xml beside the executed pom.xml.

Executed from the windows command line the project builds fine.

Some testing with command line maven showed that the .mvn folder needs to be in the directory where the mvn command is executed.
mvn -f somesubdirecotry_with_.mvn does not work
I believe mvn is not actually executed in the workspace on jenkins.

http://takari.io/2015/03/19/core-extensions.html
http://wiki.eclipse.org/Tycho/Release_Notes/0.24