File extensions in Windows are hidden by default and are typically not something you have to worry about. When you need to recover a drawing from a backup (BAK) or autosave (SV$) drawing file, you need to change the file’s extension to DWG.
- Click the Windows Start button
Windows System Run.
- In the Run dialog box, Open text box, enter control folders and then click OK.
- In the Folder Options dialog box, View tab, clear the Hide Extensions for Known File Types option under the Advanced Settings group.
- Click OK.