JavaScript HTML5: issue with drag-and-drop for a gmail-like upload interface

A screenshot of my interface:

Each element of the tree is a <li> and a <a>.

Each folder is a dropzone for file-upload like the gmail upload drag-and-drop.

When I "drag enter" on a folder with a file from my machine, I add a style:

The code for this:

var dropZoneElement = document.getElementById($this.attr('id'));

dropZoneElement.addEventListener('dragenter', onDragEnter, false);
dropZoneElement.addEventListener('dragleave', onDragLeave, false);
dropZoneElement.addEventListener('drop', onDrop, false);

function onDragEnter(event) {
  event.preventDefault();
  event.stopPropagation();
  $this.addClass('gmail-like');
}

function onDragLeave(event) {...}
function onDrop(event) {...}

But the problem is that when I drag a link (<a>) on a folder (even the fake link test with no destination, see screenshot), the page add the style to upload (no upload, it's just visual):

• Why?
• Any idea to disable this?

I would like the style applies only when I drag-and-drop a file from my machine.