Images, videos, and gifs the agent can show or reference during a conversation. Each
entry is either an update to an existing asset (provide its id), or a new item added
from your media library (provide media_id, optionally with tags) — its file, name,
description, and thumbnail come from the media library automatically. New assets can
only be added from the media library. Leave out an asset's id or media_id to remove
it from the agent.