TEX files seem to have been introduced with Cataclysm. One per continent (e.g. Azeroth.tex). Chunked structure. Internally called 'TextureBlob'.

Chunks

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)

Unknown

\0-delimited list of filenames

Rest of the file is made up with TXMD chunks