我使用SpringBoot邮件组件来生成eml文件。当我使用mockito来模拟时，为什么会生成一个真实的文件？
这是我的小样

@Resource
private JavaMailSender javaMailSender;

public File createEml(String filename) throws IOException, MessagingException {
    File file = new File(filename);
    try(FileOutputStream fos = new FileOutputStream(file)) {
        MimeMessage mimeMessage = javaMailSender.createMimeMessage();
        mimeMessage.writeTo(fos);
    }
    return file;
}

}
试验，测验

@InjectMocks
private EmailManager emailManager = new EmailManager();

@Mock
private JavaMailSender javaMailSender;

@BeforeEach
private void setUp() {
    MockitoAnnotations.openMocks(this);
}

@Test
void testCreateEml() throws MessagingException, IOException {
    MimeMessage mimeMessage = Mockito.mock(MimeMessage.class);
    Mockito.doNothing().when(mimeMessage).writeTo(Mockito.any(FileOutputStream.class));
    Mockito.when(javaMailSender.createMimeMessage()).thenReturn(mimeMessage);
    emailManager.createEml("test.eml");
}