Block Attributes
textureFileYN
This attribute defines the texture file that the block will use for its y-negative (bottom) side. Texture files for this attribute go into the textures/blocks directory of your mod.
Examples
// Uses textures/blocks/myBlockBottom.png from the mod's directory
textureFileYN[0] = "myBlockBottom.png";
// ...
// Uses textures/blocks/wood.png from the minecraft.jar
textureFileYN[15] = "/wood.png";
textureFileYN[0] = "myBlockBottom.png";
// ...
// Uses textures/blocks/wood.png from the minecraft.jar
textureFileYN[15] = "/wood.png";