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/twilio/send_on_twilio_service.rb

@@ -25,7 +25,7 @@ class Twilio::SendOnTwilioService < Base::SendOnChannelService
   end
 
   def attachments
-    message.attachments.map(&:file_url)
+    message.attachments.map(&:download_url)
   end
 
   def inbox