If I'm reading this right, I don't think it has to do with JSON. I'd suggest looking at what the URL is for the https request.
I believe that, generally, that error is because you are trying to download a file type that is not supported on the phone. It may be supported on "normal safari" (e.g. downloading an .exe or .dmg), but the phone is more selective about which types it will allow for download - since it doesn't "save to disk" to allow the user to deal with the unknown type later.
I have not looked into it, but I would guess this has to do with supported protocol handlers and/or mime types for the phone - and the URL you are requesting is not supported. :)