code: function filterFile(file, done) {
        // Ensure it matches the extension.
        const correctExtention = path.extname(file) === opts.extname
  
        // Make sure it has defined files.
        const hasFiles = files[file].files
        done(null, correctExtention && hasFiles)
      }
docs: Check if the given file is a .concat file. Call done() with result.