Heeey. JSON attributes in the API (which is released too early) is for a more flexible attributes system. In a nutshell, it’s the equivalent of using a struct as a public attribute in Unity.
We can define a data object using JSON schema (eg a texture asset, a string for a name and a number for some value) and be able to create an array of those objects.
For the OP, if you would like to refer to a JSON file in the project like Leonidas has said, use an attribute of type asset and then use asssetType JSON.
Edit: still I don’t understand why the type json is not accepted as a valid type, though Even if it is an empty field, without supplying any asset/value to it.