I'm not really sure what you are trying to do so it is hard to give a good answer. I think perhaps the dojo/on module is what you are after; this is basically an event listener module that works with DOM nodes and classes implementing dojo/Evented. For example:

require(["dojo/on"], function(on) {
    on(document.body, "hover", function(e) {
        console.log(e);
    });
});

Getting to understand how Dojo works, but lost on how to iterate through the items in the media-folder? Im going to assume that the items hasn't even been created in the doom in postCreate.

define([
    "dojo/dom",
    "dojo/query",
    "dojo/_base/declare",
    "dojo/on",
    "dijit/_WidgetBase",
    "epi/shell/_ContextMixin",
    "epi/shell/widget/_FocusableMixin",
], function (
    dom,
    query,
    declare,
    on,
    _WidgetBase,
    _ContentMixin
) {
    return declare("epilabs.imagepreview.init", [_WidgetBase, _ContentMixin], {
        //templateString: '',

        postCreate: function () {
            
            // Here?
            query(".dgrid-row").forEach(function (node) {

                console.log(node);
            });
        }
    });
});