public class Account {

    @Id
    private String _id;
    private String Username;
    private String Password;
    private String Email;
    private List<MultipartFile> Files;

@Override
public List<MultipartFile> showFiles(Account account) {
    List<MultipartFile> fileList = account.getFiles();
    return fileList;
}

@Override
public void addFile(List<MultipartFile> fileList, MultipartFile file) {
    fileList.add(file);
}