There is only one file type that uses the .WTD extension:
Grand Theft Auto IV Textures File
Texture file used by Grand Theft Auto IV (GTA4), an action/adventure game; used for texture mapping and contains a two-dimensional graphic that gives the surface appearance to a 3D object in the game; saved in the \Rockstar\Games\Grand Theft Auto IV\pc\textures\ directory.
WTD files can be edited by SparkIV or OpenIV, but the texture editing and importing process is generally only for advanced users.
Updated: November 27, 2012