There isn't any documentation anywhere but I think I can just tell you the format. Do you need color cache (basically images) or the multiplayer map files containing actual block states?
EDIT: You might have difficulties decoding the color cache because it can contain compressed data if the user has the "Texture Compression" setting enabled. The compression format is picked by the graphics driver (GL_TEXTURE_COMPRESSION_HINT set to GL_NICEST and internal format when uploading being GL_COMPRESSED_RGB). The cache files do contain the actual format used but I don't expect you to handle every format that is thrown at you.
The multiplayer map files would probably be best for my purpose.