My understanding is that in implementations any unknown type creates a "data block", which is just unprocessed text.'

I wouldn't use application/json just in case browsers start supporting that and it has different semantics than whatever custom thing you might do, causing a webcompat issue when the native feature rolls out.

Although with JSON, it's pretty unlikely that there would be any differing semantics. JSON modules in JS are just JSON blocks with no special additions and no named exports. That's what inline versions would be as well.

But wouldn't a subtype like `mytype` in my example (`application/mytype+json`) still be be a valid mime-type and still avoid your concerns? I've used these before.