After adding external user to Contacts, chat window shown as internal user chatting?!
Based on the following Microsoft document URL, file sharing is disable for external user.
And when I chat with an external user, the Files tab was not shown in the chat window.
But when I add the external user to Calling > Contacts and start a chat, Files tab is shown in the chat window. And I could upload files in the chat. (This is the problem.)
We hope that the Contacts function could distinguish the internal and external users. So that the user could not share files when chatting with external users. This will be safer for us.
Reference: Compare external and guest access
It's looks like a bug and it contains security risk. Please fix it asap.