<marquee>Я принимаю только image/jpeg <img src="http://javascript.ru/forum/images/smilies/wink.gif" /></marquee>
<script>
document.addEventListener('dragover', function(e) {
    var dataTransfer = e.dataTransfer;
    var accept = Array.prototype.some.call(dataTransfer.items, function(item){
        return item.kind == 'file' && item.type == 'image/jpeg';
    });
    dataTransfer.dropEffect = accept ? 'copy' : 'none';
    e.preventDefault();
});
</script>

var t = this;
                    t.addEventListener('dragover',function(e){
                        var data = e.dataTransfer;
                        console.log(data.items);
                        if (data.items.length > 1) {
                            data.dropEffect = 'none';
                        }
                        else if (data.items.type == 'image/jpeg') {
                            data.dropEffect = 'copy';
                        }
                        else {
                            data.dropEffect = 'none';
                        }

                    })