docs: function readExtension(file, cb) {
      fs.readFile(metapath(file), 'utf-8', function(err, data) {
          if (err) {
              if (err.code === 'ENOENT') return cb(new Error('Metadata file does not exist.'));
              return cb(err);
          }
          try {
              var ext = guessExtension(JSON.parse(data));
              if (ext) {
                  if (env == 'development') console.error("[millstone] detected extension of '" + ext + "' for '" + file + "'");
              }
              return cb(null, ext);
          } catch (e) {
              return cb(e);
          }
      });
  }