TEX files seem to have been introduced with Cataclysm. One per continent (e.g. Azeroth.tex). Chunked structure. Internally called 'TextureBlob'.
TXVR - Texture Version
As of 4.0.12479, 0
Rev 13329: 8 Byte of Data, actual only containing a 4 in the first Byte. (CataclysmCTF.tex)
TXBT - ?
TXFN - Texture Filenames
\0-delimited list of filenames
TXMD - ?
Rest of the file is made up with TXMD chunks