I haven't tested that multi texture feature in a minute now, but I never had issues with it (other than it taking quite a bit to upload everything), but as long as the file names were correctly corresponding to each respective body part they were meant to be uploaded on (such as head, l_biceps, r_foot, etc), it always worked fine for me, and maybe the file had to be .tga but don't quote me on that as I'm not 100% sure