If you want to attaching some media through your broadcast, you must enabling With Media first. Please aware for some rules if you want to uploading your files.
There is option to separating media by enabling Separate Caption. This will make your media will be sent in different bubble from your message text, and this will take effect if you just attaching only one file. There is some limitation for characters if you send media with caption (message text), please refer to the rules below message field.