MAT files are a new form of pose file that will automatically load your textures for you. bump maps, or .bum files, are what gives surface detail on a model. for example, a bump map on a face will create pore detail, raised moles, etc. theyre black and white because theyre almost like a 'terrain' - the dark parts are low, and the light parts are high. just like on a real map in an atlas. theres a tutorial on making photo textures at cake1's site: http://www.cakeone.com/