code: function filterFile(file, callback) {
        // Ensure it matches the extension.
        const correctExtention = path.extname(file) === opts.extname
  
        // Make sure it has defined files.
        const source = files[file].source || false
        const destination = files[file].destination || '.'
        callback(null, correctExtention && source && destination)
      }
docs: Check if the given file is a .concat file. Call done() with result.
  
  @param {string} file The file to filter on.
  @param {function} callback A asyncronous callback that's made once the processing is complete.