File Transfer on HTML
File Transfer is a typical feature that could appears in kinds of scenario for web applications. HTTP is simple and practical, so we use it to transfer files intead of FTP.
At front end, we need to pack file binary into HTTP packet with help of JS script and HTML FORM tag. At end front, we can use various methods and language to handle HTTP request.
前端form标签内加入file标签，发起POST请求。
HTTP报头会指明Content-Type，文件会加入到HTTP的MIME Multipart Media Encapsulation字段中。
后端cgi处理POST请求，将HTTP报文中的文件对象存储成文件。
前端对文件url发起GET请求。
后端处理GET请求，HTTP报文的Content-type标明文件类型，将文件体放在正文中返回给前端。