Custom HTML Preview
Now when you upload custom HTML to send as a Rich App Page, you have the ability to review what you've uploaded. Just click the "Preview" button:
This displays the same preview window you get with the template editor where you can test the layout in several mobile device types:
This preview function also works with duplicated messages. When you duplicate from the message history section, you will have the ability to download and edit a message with HTML. You can also click Upload to overwrite that HTML file with a new version.