In the Web Designer you can also add your own media files. If you only want to link online ressources like videos (from youtube, vimeo, etc.) you can use the show-web command.
First add the media object with the “Choose file” button and then upload it. Then you have to add the variable $systemLinkedMediaObject to a the message itself. It’s like the placeholder for the media object. At this position it will be shown. If you want to show a longer text like a 'informed consent' in html, you can use the show-info command.
Supported media files
Image files: PNG or JPEG
Audio files: AAC or M4A
Video files: MP4/H.264 or AAC audio
HTML files: HTM/HTML