dz.js

var Dropzone = (function () {
    var $dropzone, readerClbks = [];

    var handlerOf = {
        dragOver: function (e) {
            e.stopPropagation();
            e.preventDefault();
            e.dataTransfer.dropEffect = 'copy';
            $dropzone.addClass('hover');
        },

        dragLeave: function (e) {
            $dropzone.removeClass('hover');
        },

        dragDrop: function (e) {
            e.stopPropagation();
            e.preventDefault();

            $dropzone.removeClass('hover');
            processFiles(e.dataTransfer.files);
        },

        fileSelect: function (evt) {
            processFiles(evt.target.files);
        }
    };

    function processFiles(files) {
        for (var i = 0, f; f = files[i]; i++) {
            var reader = new FileReader();

            reader.onload = (function (file) {
                return function (e) {
                    readerClbks.forEach(function (callback) {
                        callback(file, e.target.result);
                    });
                };
            })(f);

            reader.readAsText(f);
        }
    }

    return {
        init: function (listeners) {
            readerClbks = listeners || [];

            var $input = document.getElementById('files');
            $input.addEventListener('change', handlerOf.fileSelect, false);

            $dropzone = document.getElementsByClassName('b-dropzone')[0];
            $dropzone.addEventListener('dragover', handlerOf.dragOver, false);
            $dropzone.addEventListener('dragleave', handlerOf.dragLeave, false);
            $dropzone.addEventListener('drop', handlerOf.dragDrop, false);
            $dropzone.addEventListener('click', function () {
                $input.click();
            }, false);
        }
    }
})();