使用 XMLHttprequest 上传文件 - multipart/form-data 中缺少边界

I'm uploading a file with XMLHttprequest. Here is the JS function, that uploads a file:

var upload = function(file) {
    // Create form data
    var formData = new FormData();
    formData.append('file', file);

    var xhr = new XMLHttpRequest();

    // Open
    xhr.open('POST', this.options.action);

    // Set headers
    xhr.setRequestHeader("Cache-Control", "no-cache");
    xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
    xhr.setRequestHeader("Content-Type", "multipart/form-data");
    xhr.setRequestHeader("X-File-Name", file.fileName);
    xhr.setRequestHeader("X-File-Size", file.fileSize);
    xhr.setRequestHeader("X-File-Type", file.type);

    // Send
    xhr.send(formData);
}

在服务器端，在 upload.php 我这样读取文件:

file_put_contents($filename, (file_get_contents('php://input')));

// Open
xhr.open('POST', this.options.action, true);

// !!! REMOVED ALL HEADERS

// Send
xhr.send(formData);