About DTX Files
The .DTX file extension primarily represents LithTech Game Engine Textures, utilized by classic titles etc F.E.A.R., No One Lives Forever, CrossFire and Wolfteam. These files store surface graphics, bump maps, and environmental skins in a format optimized for GPU rendering, often utilizing S3TC/DXT compression.
If you have installed the Vortigaunt installer, you can open the DTX file directly. Otherwise go Tools -> Lithtech -> DTX Viewer
You can export the DTX image As 8-bit BMP