It makes weapons shiny. If it's removed, the special equipment used for the storyline no longer shines, but neither do weapons which should. For example, the cutlass, with and without "EnvAmmo.tga.tx":I've tested your save game now @AkrimalS. And it is as @Grey Roger says: a special "blade"item + a standard pistol gives the shiny metal look.
I tested to lift out EnvAmmo.tga (which the standard pistols use) from the game and all works fine. (expect some error messages) So it's hard to tell what EnvAmmo does.
If the special items aren't shiny when you're not carrying a regular weapon, would it be practical to forcibly disarm the player after picking up one of the special items? You probably don't want the player switching weapons anyway. For example, if Pyle has just collected the spade, the next thing he's supposed to do is dig up the anchor, then find a use for the anchor - he shouldn't be switching to a sword and he doesn't need a pistol for any of this.
For comparison, "Hornblower" disarms the player during the scene in the Spanish prison. Hornblower is equipped with fists and cobblestone to make him look unarmed and isn't allowed to re-equip weapons until he's released from prison. It would look silly for him to be a prisoner of war and still carrying a sword and pistol!