Problem with specular map not showing in game {SOLVED}

I just tried your file in my game and the specular worked fine in CAS and in the game and when entering CAS again.

Because you still have it set as a top at the moment, the specular from other items may interfere. I would highly recommend setting the mesh as a full body outfit before you carry on so you can see how the textures will behave on the finished item. One oddity I did notive in my game with texture was that in CAS there was a very shiny panty shape on the back which I believe was from the underwear my Sim had on as the same shape is not present on your texture and it wasn't there when I changed her outfit to include leggings.

You will also want to lower the bottom skirt "cap" at the top of the legs as the upper leg inside the skirt clips through the dress on larger sized Sims.