How to allow import .json type file

Hello. I had a problem with Import Layers Style Kit Preset .Json file. I had message like “Sorry, This File Type Is Not Permitted For Security Reasons.”
On Layers documentation I read about fixing this problem - they ask to download WPMU plugin and get permission for json files. But I can't find this option on my profile. Can you help me with that? Is it right place to do that? I'm new on your platform. Thank you.