Chore: Provide fixed attachment URLs for Channels (#4507)

Prior to this change, The attachment URL sent from Chatwoot to 3rd party integrations like Whatsapp and Facebook
involved a 301 redirect before the original content is served. This causes intermittent breakages for the sent attachments.

fixes: #3632
ref: https://blog.saeloun.com/2021/09/14/rails-7-adds-expiring-urls-to-active-storage.html

app/services/facebook/send_on_facebook_service.rb

@@ -36,7 +36,7 @@ class Facebook::SendOnFacebookService < Base::SendOnChannelService
         attachment: {
           type: attachment_type(attachment),
           payload: {
-            url: attachment.file_url
+            url: attachment.download_url
           }
         }
       },