upload.js

var element = document.querySelector('.droppable');
var droppedFiles = [];
function callback(files) {
    for(var i=0; i<files.length; i++) {
        droppedFiles.push(files[i]);
    }
}
makeDroppable(element, callback);

var uploadBtn = document.querySelector('.upload-btn');
uploadBtn.addEventListener('click', function() {
  for(var i=0; i<droppedFiles.length; i++) {
      var formData = new FormData();
      formData.append("files", droppedFiles[i]);

      $.ajax({
        url: '/server_upload_url',
        method: 'post',
        data: formData,
        processData: false,
        contentType: false,
        success: function(response) {
          alert('File ' + droppedFiles[i].name + ' uploaded successfully.');
        }
      });
  }
});