return {
    acceptedFiles: "image/*",    // *** NOT WORKING ***
    init: function () {

        Dropzone.options.myDropzone = {
            init: function() {
                this.options.acceptedFiles = "image/*";

                this.on("addedfile", function(file) {
                    // Create the remove button
                    var removeButton = Dropzone.createElement("<button class='btn btn-sm btn-block'>Remove file</button>");

                    // Capture the Dropzone instance as closure.
                    var _this = this;

 [...]

Dropzone.options.myDropzone = {
    acceptedFiles: "image/*",
    init: function(){ [..] }

Dropzone.options.myDropzone = {
   init: function(){
       this.options.acceptedFiles = "image/*"
   }