correct regression uploading images

app/models/optimized_image.rb

@@ -125,8 +125,8 @@ class OptimizedImage < ActiveRecord::Base
 
   IM_DECODERS ||= /\A(jpe?g|png|tiff?|bmp|ico|gif)\z/i
 
-  def self.prepend_decoder!(path, ext_path)
-    extension = File.extname(ext_path)[1..-1]
+  def self.prepend_decoder!(path, ext_path = nil)
+    extension = File.extname(ext_path || path)[1..-1]
     raise Discourse::InvalidAccess unless extension.present? && extension[IM_DECODERS]
     "#{extension}:#{path}"
   end

spec/requests/user_avatars_controller_spec.rb

@@ -57,6 +57,9 @@ describe UserAvatarsController do
 
           expect(OptimizedImage.where(upload_id: upload.id).count).to eq(1)
           expect(response.status).to eq(200)
+
+          upload.reload
+          expect(upload.extension).to eq('png')
         end
       end