attachment (filename, options) {
    if (filename) this.type = extname(filename)
    this.set('Content-Disposition', contentDisposition(filename, options))
  }
Set Content-Disposition header to "attachment" with optional `filename`.

@param {String} [filename]
@param {object} [options]
@param {string} [options.type=attachment]
@param {string|boolean} [options.fallback=true]
@api public