cnn.dds

DirectDraw Surface texture Reference

A .dds file contains a texture in the DirectDraw Surface format. The format stores compressed image data that a graphics processor reads directly, including mipmap levels and cube maps. Games and 3D applications use it heavily. DirectX defines the format. Image tools need a plugin to open it.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.