List of Safe Add-on files

I hope I did not post this in the incorrect forum.

On to the topic. For several games, programs ect I use add-on's and user created mods. I have been warned by several of my friend that I should inspect all file types before putting the files into the modification folder, but I only know a few file types that are related to the file. I know that images files, LUA, folders and anything to that nature. Anyway, could someone post a list of file extensions that are safe and most common in user interface adds.