filter all none `.kju` files out of the array.

@param {Error} err Set when a error occurred
@param {Array} files Files to filter
@api private
function filter(err, files) {
    if (err) return self.emit('error', err);

    var kjud = files.filter(function filterfiles(file) {
      return (/(\d)\.kju$/).test(file);
    });

    // if we don't have any files we don't really need to emit 'recovered' so
    // no need to check for the length here. We are only gonna use the length
    // if there are files.
    processed = kjud.length;
    kjud.forEach(read);
  }