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

spec/services/facebook/send_on_facebook_service_spec.rb

@@ -57,6 +57,7 @@ describe Facebook::SendOnFacebookService do
         attachment = message.attachments.new(account_id: message.account_id, file_type: :image)
         attachment.file.attach(io: File.open(Rails.root.join('spec/assets/avatar.png')), filename: 'avatar.png', content_type: 'image/png')
         message.save!
+        allow(attachment).to receive(:download_url).and_return('url1')
         ::Facebook::SendOnFacebookService.new(message: message).perform
         expect(bot).to have_received(:deliver).with({
                                                       recipient: { id: contact_inbox.source_id },
@@ -70,7 +71,7 @@ describe Facebook::SendOnFacebookService do
                                                         attachment: {
                                                           type: 'image',
                                                           payload: {
-                                                            url: attachment.file_url
+                                                            url: 'url1'
                                                           }
                                                         }
                                                       },