FIX: Use ImageMagick to detect animated images (#11702)

This is a fallback when FastImage cannot be used (animated WEBP images).

spec/lib/upload_creator_spec.rb

@@ -137,6 +137,9 @@ RSpec.describe UploadCreator do
       let(:animated_filename) { "animated.gif" }
       let(:animated_file) { file_from_fixtures(animated_filename) }
 
+      let(:animated_webp_filename) { "animated.webp" }
+      let(:animated_webp_file) { file_from_fixtures(animated_webp_filename) }
+
       before do
         SiteSetting.png_to_jpg_quality = 1
       end
@@ -208,6 +211,20 @@ RSpec.describe UploadCreator do
           expect(File.extname(upload.url)).to eq('.gif')
           expect(upload.original_filename).to eq('animated.gif')
         end
+
+        it 'should not convert animated WEBP images' do
+          expect do
+            UploadCreator.new(animated_webp_file, animated_webp_filename,
+              force_optimize: true
+            ).create_for(user.id)
+          end.to change { Upload.count }.by(1)
+
+          upload = Upload.last
+
+          expect(upload.extension).to eq('webp')
+          expect(File.extname(upload.url)).to eq('.webp')
+          expect(upload.original_filename).to eq('animated.webp')
+        end
       end
     end