当我尝试使用multer上传文件时，为什么req.file对我来说是“未定义的”？

const multer = require('multer');
const COS = require('cos-nodejs-sdk-v5');

// 配置multer
const storage = multer.memoryStorage();
const upload = multer({ storage: storage });

// 创建腾讯云COS实例
const cos = new COS({
  SecretId: 'your-secret-id',
  SecretKey: 'your-secret-key',
});

// 处理文件上传
app.post('/upload', upload.single('file'), (req, res) => {
  // 获取上传的文件数据
  const file = req.file;

  if (!file) {
    return res.status(400).json({ error: 'No file provided' });
  }

  // 上传文件到腾讯云COS
  const params = {
    Bucket: 'your-bucket',
    Region: 'your-region',
    Key: file.originalname,
    Body: file.buffer,
  };

  cos.putObject(params, (err, data) => {
    if (err) {
      console.error(err);
      return res.status(500).json({ error: 'Error uploading file' });
    }

    return res.status(200).json({ success: 'File uploaded successfully' });
  });
});