code: function assetFile(filename, callback) {
        const data = {
          source: files[filename].source,
          destination: files[filename].destination || path.join(path.dirname(filename), '.')
        }
        delete files[filename]
        metalsmithAssets(data)(files, metalsmith, callback)
      }
docs: Tell Metalsmith Assets to process the data.
  
  @param {string} filename The file to filter on.
  @param {function} callback A asyncronous callback that's made once the processing is complete.