JSON plugin: Added .txt as valid file extension

This might be something only I need (#262) and might be too specific of a use case (Optimally you would just make it so you could save files as any extension in OSX but I don't know if its possible or not) but putting it out there for you to decide. This small fix allows you to save .json as .txt on OSX since it is added as a valid file extension for Json files.