return the config box or choose the right one according to the src
function getLegalBox(_ref) {
  var src = _ref.src,
      box = _ref.box;

  if (isString(box) && box) return box;
  src = src.toLowerCase();
  for (var key in boxSuffixMap) {
    var suffix = boxSuffixMap[key];
    if (src.indexOf(suffix) > -1) return key;
  }
  return 'native';
}