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.ACT
Adobe Color Table - Used to store 256-color palette data for RAW textures
.TRK
Truck File - Provides all information pertaining to how a truck behaves in-game. 
Add data from official .trk document.
.OPA
Opacity Texture - Used to provide information on what parts of the truck can be seen through, and what areas should have paint applied.
.RAW
RAW Pixel Texture - 8-bit texture data, only contains pixel values. It does not include sizing, palette data, or any attributes. Texture size is used to infer the correct sizing. NOTE: Some textures in game are hard coded to be non-square, or store multiple color channels. (ex. Loading screen/UI elements)
.TIF
Used to store textures for 4x4 Evolution 2. NOTE: it uses a non-standard TIF format that includes a 4th color channel to provide opacity texture data on trucks.