V3 sourcemaps are JSON objects [1]. Using the `application/json` mime
type makes it more obvious why this is listed under text formats.
Sourcemaps are defined to be UTF-8 so the `charset` token is
superfluous, but it shouldn't cause problems either.

See also https://stackoverflow.com/questions/19911929/

[1] https://sourcemaps.info/spec.html#h.qz3o9nc69um5