I don't think 76 is a valid modelvariation, not for TSL anyway. It lumps all the upgrade parts together under one category so there are heaps.
All you need to do for icons is to follow the existing name convention. As long as the modelvariation matches it will show up in-game.
MDL files are the models - the 3D meshes. MDX files are the model extension trees. I'm a bit hazy on what that actually consists of, but all you need to know is that they are paired.
UTI files are the templates for items. There are a whole bunch of UTx files for various things in the game. It defines the basic info about that object.