如何确定用户是否选择了文件上传文件？

const fileInput = document.querySelector('#file-input');
fileInput.addEventListener('change', (event) => {
  const files = event.target.files;
  if (files && files.length > 0) {
    console.log('用户选择了文件：', files[0].name);
  } else {
    console.log('用户没有选择文件');
  }
});

#file-input {
  display: none;
}

from flask import Flask, request

app = Flask(__name__)

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files['file']
    if file:
        # 处理文件
        pass
    else:
        # 文件未选择
        pass

if __name__ == '__main__':
    app.run()