Expose commentaire attachement on api

fix #2817

app/serializers/commentaire_serializer.rb

@@ -1,5 +1,10 @@
 class CommentaireSerializer < ActiveModel::Serializer
   attributes :email,
     :body,
-    :created_at
+    :created_at,
+    :attachment
+
+  def attachment
+    object.file_url
+  end
 end