public class X {
    @Id
    private Integer id;
    //other properties

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "field")
    private Set<Z> childs = new HashSet<>();
}

public class Z {
    @Id
    private Integer id;
    //other properties
    @Lob
    private byte[] file;
    @ManyToOne(
    @JoinColumn(name = "id", insertable = false, updatable = false))
    private X field;

    public Z(Integer id) {
         this.id = id;
    }

}

@Transactional
public void merge(X x) {
     x.getChilds().add(new Z(1));
     x.getChilds().add(new Z(2));
     entityManager.merge(x);
     entityManager.persist(x.getChilds());
}