可以上传在使用formData和express时获得未定值的文件。如何解决？

<input type="file" id="fileInput">

var formData = new FormData();
var fileInput = document.getElementById('fileInput');
formData.append('file', fileInput.files[0]);

npm install multer

var multer = require('multer');
var upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), function(req, res) {
  // 处理上传的文件
});

app.post('/upload', upload.single('file'), function(req, res) {
  var file = req.file;
  // 处理上传的文件
});