code: function (options, fileExtension) {
    var getOptionsKey = function (options, fileExtension) {
      for (var prop in options) {
        var _prop = prop.toLowerCase();
  
        if ((options[_prop].hasOwnProperty('allowed_file_extensions') && options[_prop].allowed_file_extensions.indexOf(fileExtension) > -1) || (!options[_prop].hasOwnProperty('allowed_file_extensions') && _prop === fileExtension)) {
          return prop;
        }
      }
  
      return null;
    };
  
    if (isNestedStructure(options)) {
      var optionsKey = getOptionsKey(options, fileExtension);
      return optionsKey !== null ? options[optionsKey]: {};
    }
  
    return options;
  }
docs: Method checks whether file's extension has corresponded configuration object.
  @param  {Object} options       - configuration object.
  @param  {String} fileExtension - file's extension.
  @return {String|Object}        - object key or null.