function contentDisposition$1 (filename, options) {
  var opts = options || {};

  // get type
  var type = opts.type || 'attachment';

  // get parameters
  var params = createparams(filename, opts.fallback);

  // format into string
  return format$2(new ContentDisposition(type, params))
}
lazily adds the base router if it has not yet been added.

We cannot add the base router in the defaultConfiguration because
it reads app settings which might be set after that has run.

@private