code: function(className, params) {
          var result = Utils.getUrl(config.urls.data, className + '/' + params.id + '/file');
          result += '?field_name=' + params.field_name + '&token=' + this.service.getSession().token;
          return result;
      }
docs: Return file's URL from file field by ID
  @memberof QB.data
  @param {string} className - A class name of record
  @param {object} params - Object of parameters
  @param {string} params.field_name - The file's field name
  @param {string} params.id - The record's ID