Appendix C. AutoCAD File Extensions
AutoCAD uses a lot of different file extensions to differentiate files—by some counts nearly 200. The list in this table includes those most specific to AutoCAD, or that I think you're most likely to run into. Many other file types can be used by AutoCAD for various purposes, including image files used to create materials and button images, and hypertext files.
3D Studio. This is created by the 3DSOUT command for use in 3D Studio VIZ or MAX.
Extensions that contain a dollar sign are temporary files. The .ac$ file extension is given to AutoCAD temporary files that are used to store data when RAM is filled. They should be deleted when AutoCAD closes, if it closes without crashing. If ...