How do I add a new mockup category?

Hi guys, I am just wondering which file/files I need to modify to add a new mockup category, As I previously tried but, it showed an internal error occured, report a bug ?. SO, just came to the community so that I can be guided about where to start

For those who are confused about what I want to add,


I want to add a category among this

Its located in

Clear more about this ??

did you check this -