getFiles(callback) {
    get(this.getFileListUrl(this.type, this.config), (success, error) => {
      if (error) callback(success, error);
      // find the file elements that are valid files, exclude others
      this.getFileElements(success).forEach((file) => {
        var fileUrl = this.getFileUrl(file, this.config.mode, this.type);
        if (isValidFile(fileUrl, this.config.extension)) {
          this.files.push(new File(fileUrl, this.type, this.layout.single, this.config));
        }
      });
      callback(success, error);
    });
  }
Get files from file listing and set to file collection.
@method
@async
@param {function} callback - Callback function