fix: editing buttons missing in admin when file present

Fixes #1499

Author: pajowu

filer/templates/admin/filer/widgets/admin_file.html

@@ -14,7 +14,7 @@
         </span>
     </div>
 
-    <div class="js-filer-dropzone filer-dropzone{% if object %} js-object-attached{% endif %}"
+    <div class="js-filer-dropzone filer-dropzone{% if object %} js-object-attached image-preview-container{% endif %} "
          data-url="{% url 'admin:filer-ajax_upload' %}"
          data-max-files="1"
          {% if max_filesize %}data-max-filesize="{{ max_filesize|safe }}"{% endif %}>