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

lib/integrations/slack/send_on_slack_service.rb

@@ -73,7 +73,7 @@ class Integrations::Slack::SendOnSlackService < Base::SendOnChannelService
   end
 
   def file_type
-    File.extname(message.attachments.first.file_url).strip.downcase[1..]
+    File.extname(message.attachments.first.download_url).strip.downcase[1..]
   end
 
   def file_information