上传文件的reactjs中formData.append为空值

import React, { useState } from 'react';

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

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

  const handleSubmit = async (event) => {
    event.preventDefault();

    if (!file) {
      alert('请选择一个文件');
      return;
    }

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

    try {
      const response = await fetch('/upload', {
        method: 'POST',
        body: formData,
      });

      if (response.ok) {
        alert('文件上传成功');
      } else {
        alert('文件上传失败');
      }
    } catch (error) {
      console.error('上传文件时出错:', error);
    }
  };

  return (
    <form onSubmit={handleSubmit}>
      <input type="file" onChange={handleFileChange} />
      <button type="submit">上传</button>
    </form>
  );
}

export default FileUpload;