RTF is a global modification that replaces all towns in the game.

So, it's about the glossiness/matte finish of this mirror texture, right?
But it still won't replace the normal map....

I've written about this a million times, but it seems relevant here. When I was modding Attila, the texture set included: diffuse (the actual color), normal map (for all those small details and patterns), gloss map (which seems to be analogous to this mirror effect), and specular (something related to shine, used as an addition to the gloss map)....
In Heroes 5, it's definitely more simplified.