numerous updates

static/filer/js/addons/popup_handling.js

@@ -33,6 +33,7 @@ if (django.jQuery) {
         element.val(chosenId);
         element.closest('.js-filer-dropzone').addClass('js-object-attached');
         image.attr('src', chosenThumbnailUrl).removeClass('hidden');
+        image.removeAttr('srcset');  // would be nicer, but much more complicate to also replace 'srcset'
         descriptionText.text(chosenDescriptionTxt);
         clearer.removeClass('hidden');
         lookup.addClass('related-lookup-change');