如果您避开，浏览器会设置正确的标题（包括Content-type中正确的多部分边界指示）不要手动指定任何东西。

The browser sets the correct headers (including the correct multipart boundary indication in the Content-type) if you haven't manually specified anything.

所以你需要做的就是删除以下行：

So all you need to do is remove the following line:

 hr.setRequestHeader("Content-type", "multipart/form-data");

否则你需要自己设置边界，正如Ellias Van Ootegem在使用XMLHttprequest上传文件 - 多部分/表格中缺少边界 - 数据：

Otherwise you will need to set the boundary yourself as it is suggested by Ellias Van Ootegem in Uploading a file with XMLHttprequest - Missing boundary in multipart/form-data:

hr.setRequestHeader("Content-type","multipart/form-data; charset=utf-8; boundary=" + Math.random().toString().substr(2));