fix: Export attachments url in proposal serializer

lib/extends/serializers/proposals/proposals_serializer_extends.rb

@@ -33,6 +33,7 @@ def serialize
       },
       comments: proposal.comments_count,
       attachments: proposal.attachments.count,
+      attachment_urls: attachment_urls,
       followers: proposal.followers.count,
       published_at: proposal.published_at,
       url: url,
@@ -45,6 +46,12 @@ def serialize
       }
     }
   end
+
+  def attachment_urls
+    proposal.attachments.map do |attc|
+      Rails.application.routes.url_helpers.rails_blob_url(attc.file.blob, host: proposal.participatory_space.organization.host)
+    end.join(",")
+  end
 end
 
 Decidim::Proposals::ProposalSerializer.class_eval do