An HTTP post including a file link will show up as plain text in RingCentral messaging app.
However, sending this exact text manually (via the RingCentral app) will create the appropriate link. For example:
Additionally, sending a web link via an http post works as well. For example:
The documentation here (https://developers.ringcentral.com/guide/team-messaging/manual/formatting) seems to imply that mini-markdown should be applied and I should, in fact, be able to create a clickable link to a file this way.