Extract image out of a list item

Question

I have a ul with li of images. I am taking the last li and want to extract the image data from it to upload, other wise the server (Flask) tells me that the data is an HTMLLIElement.

So how do I got about this? Here's the JS code:

        // e.preventDefault(e);
        var img = new FormData();
        img.append('file',$("#preview_list li:last-child")[0]);

        // Debugging statement
        //$("#preview_list li:last-child").dialog();
        $.ajax({
            type : "POST",
            url : "/_process",
            data : img,
            contentType: false,
            dataType: false,
            cache: false,
            processData: false,
            success: function(data,textStatus,jqXHR) {
                alert(data['result']);
            }
        });