drag and drop uploads

a.js

if (isAdvancedUpload) {

  var droppedFiles = false;

  $form.on('drag dragstart dragend dragover dragenter dragleave drop', function(e) {
    e.preventDefault();
    e.stopPropagation();
  })
  .on('dragover dragenter', function() {
    $form.addClass('is-dragover');
  })
  .on('dragleave dragend drop', function() {
    $form.removeClass('is-dragover');
  })
  .on('drop', function(e) {
    droppedFiles = e.originalEvent.dataTransfer.files;
  });

}


if (isAdvancedUpload) {
  e.preventDefault();

  var ajaxData = new FormData($form.get(0));

  if (droppedFiles) {
    $.each( droppedFiles, function(i, file) {
      ajaxData.append( $input.attr('name'), file );
    });
  }

  $.ajax({
    url: $form.attr('action'),
    type: $form.attr('method'),
    data: ajaxData,
    dataType: 'json',
    cache: false,
    contentType: false,
    processData: false,
    complete: function() {
      $form.removeClass('is-uploading');
    },
    success: function(data) {
      $form.addClass( data.success == true ? 'is-success' : 'is-error' );
      if (!data.success) $errorMsg.text(data.error);
    },
    error: function() {
      // Log the error, show an alert, whatever works for you
    }
  });
}

references: https://css-tricks.com/drag-and-drop-file-uploading/