要设置内容类型，您需要修改

In order to set the content type you need to modify the request header.

我不知道您要设置哪种内容类型，所以我默认使用json。

I don't know which content type you'd like to set, so I've defaulted to json.

以下是设置方法：

var fd = new FormData();
fd.append("upload-file", document.getElementById('upload-file').files[0]);

var xhr = new XMLHttpRequest();

/* I add some event listeners here for progress, load, error, and abort */

xhr.open("POST", "upload.php");
xhr.setRequestHeader("Content-type","application/json");
xhr.send(fd);

编辑： 从MDN设置HTTP请求标头