如何使用包含文件和发布数据的 JavaScript 创建 AJAX 请求

I have found the following suggestion but it does not seem to be complete

xhr.open("POST", "upload.php");
var boundary = '---------------------------';
boundary += Math.floor(Math.random()*32768);
boundary += Math.floor(Math.random()*32768);
boundary += Math.floor(Math.random()*32768);
xhr.setRequestHeader("Content-Type", 'multipart/form-data; boundary=' + boundary);
var body = '';
body += 'Content-Type: multipart/form-data; boundary=' + boundary;
//body += '
Content-length: '+body.length;
body += '

--' + boundary + '
' + 'Content-Disposition: form-data; name="';
body += 'myfile"; filename="'+file.fileName+'" 
';
body += "Content-Type: "+file.type;
body += '

';
body += file.getAsBinary();
body += '
'
body += '--' + boundary + '
' + 'Content-Disposition: form-data; name="submitBtn"

Upload
';
body += '--' + boundary + '--';
xhr.setRequestHeader('Content-length', body.length);

为了让这个工作，我需要一个包含输入类型文件字段的文件"变量，但是在哪里放置额外的帖子数据?我也想发送描述文本.假设我还需要使用 xhr.send 来发送请求...

Content-Type: multipart/form-data; boundary=AaB03x

--AaB03x
Content-Disposition: form-data; name="submit-name"

Larry
--AaB03x
Content-Disposition: form-data; name="files"; filename="file1.txt"
Content-Type: text/plain

... contents of file1.txt ...
--AaB03x--