正在将文件从React应用程序传递到Node应用程序以上载到S3，定义上载参数时文件是否未定义？

import React, { useState } from 'react';
import axios from 'axios';

const FileUpload = () => {
  const [file, setFile] = useState(null);

  const handleFileChange = (event) => {
    setFile(event.target.files[0]);
  };

  const handleUpload = () => {
    const formData = new FormData();
    formData.append('file', file);

    axios.post('/upload', formData)
      .then(response => {
        console.log('File uploaded successfully');
      })
      .catch(error => {
        console.error('Error uploading file', error);
      });
  };

  return (
    <div>
      <input type="file" onChange={handleFileChange} />
      <button onClick={handleUpload}>Upload</button>
    </div>
  );
};

export default FileUpload;

const express = require('express');
const multer = require('multer');
const AWS = require('aws-sdk');

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

app.post('/upload', upload.single('file'), (req, res) => {
  const file = req.file;

  // 使用AWS SDK将文件上传到S3
  const s3 = new AWS.S3();
  const params = {
    Bucket: 'your-bucket-name',
    Key: file.originalname,
    Body: file.buffer,
  };

  s3.upload(params, (error, data) => {
    if (error) {
      console.error('Error uploading file to S3', error);
      res.status(500).send('Error uploading file');
    } else {
      console.log('File uploaded to S3 successfully');
      res.send('File uploaded');
    }
  });
});

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