feat: API to list all attachments for a conversation (#7059)

Fixes: https://linear.app/chatwoot/issue/CW-1678/api-to-list-all-attachments-for-a-conversation

spec/factories/messages.rb

@@ -20,6 +20,13 @@ FactoryBot.define do
       end
     end
 
+    trait :with_attachment do
+      after(:build) do |message|
+        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')
+      end
+    end
+
     after(:build) do |message|
       message.sender ||= message.outgoing? ? create(:user, account: message.account) : create(:contact, account: message.account)
       message.inbox ||= message.conversation&.inbox || create(:inbox, account: message.account)