Dropzone is not defined

Your code might run too soon. Wrap it in:

window.onload = function() {
    // access Dropzone here
};

or, better (runs sooner than above code):

document.addEventListener("DOMContentLoaded", function() {
    // access Dropzone here
});

or, if you use jQuery:

$(function() {
    // access Dropzone here
});

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