Passing Variable from javascript(Ajax) to PHP

I have got a script for file uploading with progress bar . I have done some modifications to do some check and everything working perfectly. Just have a little problem about passing a variable from the javascript(ajax) file to the php file

here is my code:

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);
});

The form of sending is already set but can't really modify it well because didn't work with javascript much. All i try to do is to pass var folder = 7; to upload.php as seen in this part of code :

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

Any idea how to pass the variable?