Why Slack's Now Virtual Agent was auto remove servicenow's link in response (html, image, card)

[Lien Nguyen]

I try to response image as below format to Slack base on Now Virtual Agent.

<img src="instanceXXX.service-now.com/sys_attachment.do?sys_id=b04bdab81b14b81014293265cc4bcbd0"/> This is servicenow image 

But only Text responsed ("This is servicenow image"), image/link was removed. This only happen on image/link with src contain servicenow's Instance link.

Anyone know reason ? Please help me