MapperUtil cannot load mapper.properties (and then can't create mapper)

Attempting to run a test case for a mojo that uses file-management, with a mapper defined by <type>regexp</type> I get the exception

java.lang.IllegalStateException: Cannot find classpath resource: mapper.properties
at org.apache.maven.shared.model.fileset.mappers.MapperUtil.initializeBuiltIns(MapperUtil.java:64)
at org.apache.maven.shared.model.fileset.mappers.MapperUtil.getFileNameMapper(MapperUtil.java:98)
at org.apache.maven.shared.model.fileset.util.FileSetManager.mapIncludedFiles(FileSetManager.java:162)
... 26 more

Looking at file-management-1.2.1.jar I can see a file called mappers.properties but no mapper.properties. Copying the file into my own project and changing the file name means that this exception no longer occurs.

Instead, there is a NullPointerException caused by the implementations variable not being set during initializeBuiltins()

Changing to the classname property and setting the type to null (since it is otherwise intialised to "identity") could be a workaround. However, the RegexpPatternMapper seems to be missing from the .jar

The mappings provided by mapping.properties refer to classes in the org.apache.maven.plugin.assembly.mappers package, which doesn't appear to be anywhere.