Extensions are hidden by default -- and why turning them on is worth it
OS makers hide extensions by default to keep the interface simpler for casual users, but security-conscious users and anyone managing lots of files benefit from turning them back on, since a hidden double extension (like "invoice.pdf.exe") is a classic disguise trick for malware.
The setting is system-wide, not per-folder
On both Windows and Mac, this is a single global setting that applies to every folder and drive at once, not something you configure separately for each location.
Frequently Asked Questions
If I accidentally delete a file's extension while renaming it, what happens?
The file usually stops opening with its default program until you manually retype the correct extension, since the operating system uses it to know which app should open the file.
Does showing extensions change the file itself?
No, it only changes how file names are displayed on your screen -- the underlying files themselves are completely unaffected.