重点：multipart/form-data 后端识别上传类型必填

1. form 表单上传

<!-- enctype="multipart/form-data" 这个必填 -->
<form 
    action="http://127.0.0.1:8080/users/avatar" 
    method="post" 
    enctype="multipart/form-data"
>
  <input name="name" type="text" />
  <input id="file" name="files" type="file" />
  <button>提交</button>
</form>
<script>
  const upload = document.getElementById("file");
  upload.onchange = function (value) {
    const fileList = value.target.files;
    console.log(fileList);
  };
</script>

2. js 上传

// 后端上传接口
function updateAvatar(data) {
  return api({
    url: "/users/avatar",
    method: "post",
    // 设置请求头(重点)
    headers: {
      "Content-Type": "multipart/form-data",
    },
    data,
  });
}

let cutDown = (data) => {
  // 构建 form 表单;
  let formData = new FormData();
  let { file } = data;
  console.log(data);
  // file 参数命名和后端一致
  formData.append("file", file);
  updateAvatar(formData).then((res) => {
    console.log(res);
  });
};