jQuery.on("drop") not firing

You need to cancel all events

$("html").on("dragover", function(event) {
    event.preventDefault();  
    event.stopPropagation();
    $(this).addClass('dragging');
});

$("html").on("dragleave", function(event) {
    event.preventDefault();  
    event.stopPropagation();
    $(this).removeClass('dragging');
});

$("html").on("drop", function(event) {
    event.preventDefault();  
    event.stopPropagation();
    alert("Dropped!");
});

In addition to Christian's solution this can be shortened to:

$('#my-dropzone')
    // crucial for the 'drop' event to fire
    .on('dragover', false) 

    .on('drop', function (e) {
        // do something
        return false;
    });