setupDrop: bind the 'drop' event for a DOM element
function setupDrop(dropbox, opts) {

        if (!FileReaderJS.enabled) {
            return;
        }
        var instanceOptions = extend(extend({}, FileReaderJS.opts), opts);
        var dragClass = instanceOptions.dragClass;
        var initializedOnBody = false;

        // Bind drag events to the dropbox to add the class while dragging, and accept the drop data transfer.
        dropbox.addEventListener("dragenter", onlyWithFiles(dragenter), false);
        dropbox.addEventListener("dragleave", onlyWithFiles(dragleave), false);
        dropbox.addEventListener("dragover", onlyWithFiles(dragover), false);
        dropbox.addEventListener("drop", onlyWithFiles(drop), false);

        // Bind to body to prevent the dropbox events from firing when it was initialized on the page.
        document.body.addEventListener("dragstart", bodydragstart, true);
        document.body.addEventListener("dragend", bodydragend, true);
        document.body.addEventListener("drop", bodydrop, false);

        function bodydragend(e) {
            initializedOnBody = false;
        }

        function bodydragstart(e) {
            initializedOnBody = true;
        }

        function bodydrop(e) {
            if (e.dataTransfer.files && e.dataTransfer.files.length ){
                e.stopPropagation();
                e.preventDefault();
            }
        }

        function onlyWithFiles(fn) {
            return function() {
                if (!initializedOnBody) {
                    fn.apply(this, arguments);
                }
            };
        }

        function drop(e) {
            e.stopPropagation();
            e.preventDefault();
            if (dragClass) {
                removeClass(dropbox, dragClass);
            }
            processFileList(e, e.dataTransfer.files, instanceOptions);
        }

        function dragenter(e) {
            e.stopPropagation();
            e.preventDefault();
            if (dragClass) {
                addClass(dropbox, dragClass);
            }
        }

        function dragleave(e) {
            if (dragClass) {
                removeClass(dropbox, dragClass);
            }
        }

        function dragover(e) {
            e.stopPropagation();
            e.preventDefault();
            if (dragClass) {
                addClass(dropbox, dragClass);
            }
        }
    }