Fix: JSON Content Type has file extension .js

Not sure if this is intentional (will probably work), but the Resource content type JSON results in a file extension / url extension of .js and not as I would expect in .json (if I want .js I would choose the JavaScript content type, I guess...)

This was implemented here: https://github.com/modxcms/revolution/commit/08090b3f2ea39f2eab5eaf4d3c710e674a7e7518