我正在尝试通过我创建的API端点控制器上载文件：

/**
 * @Route("/upload", methods="POST")
 */
public function upload(Request $request)
{
    $form = $this->createForm(UserFileType::class);
    $form->handleRequest($request);

    if (!$form->isSubmitted()) {
        dd($request->files->get('file'));
    }
   ...

dd($request->files->get('file'))按预期显示我的文件，所以我不清楚为什么当方法接收multipart/form-dataPOST请求和数据时，isSubmitted()返回false。我通过Postman提交POST请求。为什么表单没有提交？
用户文件类型：

class UserFileType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('file', FileType::class, [
                'mapped' => false,
                'required' => true,
                'constraints' => [
                    new File([
                        'maxSize' => '2M',
                        'mimeTypes' => [
                            'application/pdf',
                            'application/x-pdf',
                        ],
                        'maxSizeMessage' => 'The file size must not exceed {{ limit }} {{ suffix }}.',
                        'mimeTypesMessage' => 'The file type {{ type }} is not valid',
                    ])
                ],
            ]);
    }