An image from a URL, in the text (type "image"): Markdown's
. The JSON keys are url, alt and title.
An image that a person uploads is a Kotoba.Nodes.Attachment; this node
is the image of Markdown, a URL that the document links to.
The URL is checked with Kotoba.Sanitizer.link_url/2. An image with a URL
that is not safe, or under the :untrusted policy (which loads no file
from a URL), renders as its alternative text.