code: function(args) {
        if (!args.src) {
          ret = new Error('metalsmith-convert: "src" arg is required');
          return;
        }
  
        if (!args.target) { // use source-format for target in convertFile
          args.target = '__source__';
        }
        var ext = args.extension || '.' + args.target;
  
        if (!args.nameFormat) {
          if (args.resize) {
            args.nameFormat = '%b_%x_%y%e';
          } else {
            args.nameFormat = '%b%e';
          }
        }
        if (!!args.remove // don't remove source-files from build if 
            && args.nameFormat === '%b%e' // the converted target has the
            && args.target === '__source__') { // same name
          args.remove = false;
        }
        Object.keys(files).forEach(function (file) {
          convertFile(file, ext, args, files, results);
        });
      }
docs: what to return;