code: function concatenateFile(file, done) {
        // Append the concat file itself to the end of the concatenation.
        files[file].files.push(file)
  
        // Make sure the output is defined.
        if (!Object.prototype.hasOwnProperty.call(files[file], 'output')) {
          // Output the file to the destination, without the ".concat".
          const dir = path.dirname(file)
          const filename = path.basename(file, opts.extname)
          const final = path.join(dir, filename)
          files[file].output = final
        }
  
        // Tell Metalsmith Concat plugin to concatinate the files.
        metalsmithConcat(files[file])(files, metalsmith, done)
      }
docs: Tell Metalsmith Concatenate to process on the given file config.