zh-CN
点击上传用户头像，并使用 beforeUpload 限制用户上传的图片格式和大小。
beforeUpload 的返回值可以是一个 Promise 以支持异步处理，如服务端校验等：示例。
en-US
Click to upload user's avatar, and validate size and format of picture with beforeUpload.
The return value of function beforeUpload can be a Promise to check asynchronously. demo
