将变量从javascript（Ajax）传递到PHP

我有一个用于进度条上传文件的脚本。我做了一些修改以进行检查，然后一切正常。关于将变量从javascript（ajax）文件传递到php文件有一点问题

这是我的代码：

var handleUpload = function(event){
    event.preventDefault();
    event.stopPropagation();
    var folder = 7;
    var fileInput = document.getElementById('file');

    var data = new FormData();

    data.append('ajax', true);
    for (var i = 0; i < fileInput.files.length; ++i){
        data.append('file[]', fileInput.files[i]);
    }   

    var request = new XMLHttpRequest();

    request.upload.addEventListener('progress', function(event){
        if(event.lengthComputable){
            var percent = event.loaded / event.total;
            var progress = document.getElementById('upload_progress');

            while (progress.hasChildNodes()){
                progress.removeChild(progress.firstChild);
            }
            progress.appendChild(document.createTextNode(Math.round(percent * 100) +' %'));
            document.getElementById("loading-progress-17").style.width= Math.round(percent * 100) +'%';
        }
    });
    request.upload.addEventListener('load', function(event){
        document.getElementById('upload_progress').style.display = 'none';
    });
    request.upload.addEventListener('error', function(event){
        alert('Upload failed');
    });
    request.addEventListener('readystatechange', function(event){
        if (this.readyState == 4){
            if(this.status == 200){
                var links = document.getElementById('uploaded');
                var uploaded = eval(this.response);
                var div, a;
                for (var i = 0; i < uploaded.length; ++i){
                    div = document.createElement('div');
                    a = document.createElement('a');
                    a.setAttribute('href', 'files/' + uploaded[i]);
                    a.appendChild(document.createTextNode(uploaded[i]));
                    div.appendChild(a);
                    links.appendChild(div);
                }
            }else{
                console.log('server replied with HTTP status ' + this.status);
            }
        }
    });
    request.open('POST', 'upload.php');
    request.setRequestHeader('Cache-Control', 'no-cache');
    document.getElementById('upload_progress').style.display = 'block';
    request.send(data);

}

window.addEventListener('load', function(event){

    var submit = document.getElementById('submit');
    submit.addEventListener('click', handleUpload);
});

发送的形式已经设置好了，但由于不能很好地使用javascript，因此无法真正进行修改。我试图做的就是传递var folder = 7;到upload.php，如本部分代码所示：

request.open('POST', 'upload.php');
request.setRequestHeader('Cache-Control', 'no-cache');
document.getElementById('upload_progress').style.display = 'block';
request.send(data);

任何想法如何传递变量？