fix

src/project/models.py

@@ -342,4 +342,8 @@ class Gallery(CMSPlugin):
         verbose_name_plural = 'Gallerien'
 
     def __str__(self):
-        return self.folder.name
+        return self.folder.name
+
+    @property
+    def files(self):
+        return self.folder.files.all().order_by('name')

src/project/templates/project/plugins/content/gallery.html

@@ -1,7 +1,7 @@
 {% load cms_tags i18n thumbnail %}
 
 <div class="gallery" itemscope itemtype="http://schema.org/ImageGallery">
-    {% for file in instance.folder.files %}
+    {% for file in instance.files %}
         <div class="gallery__item">
             {% thumbnail file 2000x2000 as full %}
             {% thumbnail file 500x500 as thumb %}