使用Fetch将文件从React前端发送到Express 4 API时出现问题

const formData = new FormData();
formData.append('file', file); // 将文件添加到FormData中

fetch('/api/upload', {
  method: 'POST',
  body: formData,
  headers: {
    'Content-Type': 'multipart/form-data', // 设置请求头为multipart/form-data
  },
})
  .then(response => response.json())
  .then(data => {
    // 处理服务器返回的响应数据
  })
  .catch(error => {
    // 处理请求错误
  });

const express = require('express');
const multer = require('multer');

const app = express();
const upload = multer({ dest: 'uploads/' }); // 配置multer中间件

app.post('/api/upload', upload.single('file'), (req, res) => {
  // 处理接收到的文件
  const file = req.file;
  
  // 返回响应
  res.json({ message: '文件上传成功' });
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

const express = require('express');
const cors = require('cors');

const app = express();

app.use(cors()); // 使用cors中间件处理跨域请求

// 路由配置
...

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});