Is there a file extension and MIME type for Unity Rich Text

When getting Unity Rich Text from an external source, what should we use?

At the moment, I’m thinking .u3dml and text/x-u3dml (which I suppose would be text/vnd.u3dml if it was officially sanctioned by Unity).

I don’t want to use .html text/html or .xml text/xml since the Unity syntax isn’t valid HTML or XML, plus my API will return actual HTML for text/html.

No, there is no file extension and MIME type. Does it actually matter what file extension you use on your server? Also, if you invented your own text/Golfenstein3D MIME type, your code can check that your data is transferred using this, but any other machine in internet-land that tried to access your data won’t know what the MIME type means, so may not know how to handle the transfer. What problem would that cause? Probably nothing, and you may not want people accessing these data outside of your app.