Fix #3952: Adding animated_gif tag to a ugoira post breaks the player.

test/unit/post_test.rb

@@ -1140,6 +1140,16 @@ class PostTest < ActiveSupport::TestCase
         end
       end
 
+      context "tagged with animated_gif or animated_png" do
+        should "remove the tag if not a gif or png" do
+          @post.update(tag_string: "tagme animated_gif")
+          assert_equal("tagme", @post.tag_string)
+
+          @post.update(tag_string: "tagme animated_png")
+          assert_equal("tagme", @post.tag_string)
+        end
+      end
+
       should "have an array representation of its tags" do
         post = FactoryBot.create(:post)
         post.reload