下面的代码，直接存为html，在浏览器中打开
<html> <header></header> <body> <input type="file"> </body> <footer></footer> </html>
显示结果如下所示，
自动带有浏览按钮
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file
<input> elements with type="file" let the user choose one or more files from their device storage. Once chosen, the files can be uploaded to a server using form submission, or manipulated using JavaScript code and the File API.
客户端选择文件之后，有三种方式将文件上传到服务器，