fix: avoid Slack file upload API for fallback messages (#10461)

Skip calling the Slack file upload API for message types such as
fallback (e.g., Facebook and location messages) that lack actual file
data in attachments. This prevents unnecessary API calls and resolves a
Sentry error currently occurring in production.

fixes: https://github.com/chatwoot/chatwoot/issues/10460

spec/lib/integrations/slack/send_on_slack_service_spec.rb

@@ -180,6 +180,28 @@ describe Integrations::Slack::SendOnSlackService do
         expect(message.attachments).to be_any
       end
 
+      it 'will not call file_upload if attachment does not have a file (e.g facebook - fallback type)' do
+        expect(slack_client).to receive(:chat_postMessage).with(
+          channel: hook.reference_id,
+          text: message.content,
+          username: "#{message.sender.name} (Contact)",
+          thread_ts: conversation.identifier,
+          icon_url: anything,
+          unfurl_links: true
+        ).and_return(slack_message)
+
+        message.attachments.new(account_id: message.account_id, file_type: :fallback)
+
+        expect(slack_client).not_to receive(:files_upload)
+
+        message.save!
+
+        builder.perform
+
+        expect(message.external_source_id_slack).to eq 'cw-origin-6789.12345'
+        expect(message.attachments).to be_any
+      end
+
       it 'sent a template message on slack' do
         builder = described_class.new(message: template_message, hook: hook)
         allow(builder).to receive(:slack_client).and_return(slack_client)