How can I fix this "Dropzone already attached" error?

You should use either

var myDropzone = new Dropzone("div#myDrop", { url: "/file/post"});

or

$("div#myDrop").dropzone({ url: "/file/post" });

not both. Basically what you are doing is calling the same thing twice.

Defining below code globally will help:

Dropzone.autoDiscover = false;

Add Dropzone.autoDiscover = false before $(document).ready like this:

Dropzone.autoDiscover = false;
$(document).ready(function () {

});

This error can also happen when an element has already had a class dropzone.

However if this is removed then for some reason the default style will not apply after Dropzone is initiated. My only workaround is to create a custom style for that element.