index.js
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)
    }
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)
    }
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 ) }
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.
Tell Metalsmith Assets to process the data .