Split file with JavaScript or jQuery

I need to upload a part of a file (only the first MB). I've created a PHP script which uploads the whole file. The data (formData Object) is passed by a ajax call.

My idea would be now to split the file with javascript (jquery). Is there any solution for my request?

Current code:

function start(a){
    //var fSize = $('#fileUpload')[0].files[0].size / 1024;
    var formData = new FormData();    
    formData.append( 'fileUpload', $('#fileUpload')[0].files[0] );
    //AJAX
    $.ajax({
        url: 'script.php',
        type: 'POST',
        data: formData,
        processData: false,
        contentType: false,
        success: function(msg){
            alert("Win: " + msg);
        },
        error: function(bla, msg){
            alert("Fail: " + msg);
        }
    });
}