Microsoft has gradually gotten less stupid with their security, but one persistent bonehead default setting persists: hiding file extensions. This lamebrained setting means that someone could send you what appears to be a harmless text file, but when you double-click it to view, it is actually a trojan executable that harms your system. To view file extensions:
1. Right-click the Start button and open the Control Panel.
2. In the Control Panel, open File Explorer Options (if your Control Panel is set to Category view, you’ll find this in Appearance and Personalization > File Explorer Options).
3. In the File Explorer Options windows, click the View tab and find the box that says Hide extensions for known file types. By default, this box will be checked. Uncheck it and click OK to apply your changes — all files in the File Explorer will now have visible file extensions.