fix #2237

app/models/post.rb

@@ -137,6 +137,14 @@ class Post < ActiveRecord::Base
       file_ext =~ /jpg|jpeg|gif|png/i
     end
 
+    def is_animated_gif?
+      if file_ext =~ /gif/i
+        return Magick::Image.ping(file_path).length > 1
+      else
+        return false
+      end
+    end
+
     def is_flash?
       file_ext =~ /swf/i
     end
@@ -550,7 +558,7 @@ class Post < ActiveRecord::Base
     def add_automatic_tags(tags)
       return tags if !Danbooru.config.enable_dimension_autotagging
 
-      tags -= %w(incredibly_absurdres absurdres highres lowres huge_filesize flash webm)
+      tags -= %w(incredibly_absurdres absurdres highres lowres huge_filesize animated_gif flash webm)
 
       if has_dimensions?
         if image_width >= 10_000 || image_height >= 10_000
@@ -579,6 +587,10 @@ class Post < ActiveRecord::Base
         tags << "huge_filesize"
       end
 
+      if is_animated_gif?
+        tags << "animated_gif"
+      end
+
       if is_flash?
         tags << "flash"
       end