A map texture is simply a 256x1024 bitmap with 4 bits per pixel. Each byte stores two pixels, with the left-hand pixel being stored in the lower 4 bits. There is no other data in map texture files.
Texture files can be identified by their size, which is always 131072 bytes (256 * 1024 * 4 / 8). They can also be identified by scanning the GNS files in the MAP directory: any file with 0x1701 in field D is a texture.
Palettes for map textures are stored in Mesh files.