How do I see file extensions, and why does it matter?
A three-letter setting that tells you what a file actually is. Turning it on takes a moment and prevents a whole class of mistakes, including a common attachment trick.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
- Safety
- warning
Short answer
Both systems hide the extension by default and both let you turn it on: on Windows, in the file browser's view options; on macOS, in the Finder's advanced settings. Turn it on. You will then see that a photo is a .heic rather than a .jpg, that an attachment is a .exe rather than a .pdf, and why a file will not open in the program you expected. The one rule afterwards is not to change an extension expecting it to convert the file.
Safety
Step by step
- Turn it on.Windows: the view section of the file browser has a tick for file name extensions. macOS: the Finder's settings has an option to show all filename extensions. Both are one tick and apply everywhere immediately.
- Understand what the extension does.It is the last part of the name after the dot, and it is how the system decides which program opens the file. It is part of the name, not a property of the contents, which is why it can be wrong or misleading.
- Use it to know what you have..docx is a modern Word document and .doc an old one; .jpg and .heic are both photographs but only one opens everywhere; .csv and .xlsx are both spreadsheets but behave very differently; .zip is a container holding other files. The extension answers the question of why something will not open.
- Use it to spot a dangerous attachment.With extensions hidden, a file called statement.pdf.exe appears as statement.pdf and the icon can be made to match. With them shown, the .exe is visible. This is one of the oldest and still most effective tricks in email fraud.
- Know that renaming does not convert.Changing a .heic to .jpg does not convert the photograph; it mislabels it, and the file will then fail to open or open incorrectly. Conversion requires software that reads one format and writes the other.
- Know when renaming is legitimate.Occasionally a file has the wrong extension — a .txt that is really a .csv, a .jpeg that a program insists on calling something else. Correcting a genuinely wrong label is fine. Changing a correct one is not.
- Take care when renaming with extensions visible.Once shown, the extension is part of what you are editing, so renaming a file can accidentally remove or change it. Both systems will warn you; take the warning seriously.
- Use it when filing.Sorting a folder by type, or searching for an extension, becomes far more useful once you can see what you are dealing with. It is the difference between managing a folder and guessing at it.
Questions people ask
Why are extensions hidden by default?
To make the interface look simpler, and to stop people breaking files by renaming them. The security cost of hiding them is, on balance, worse than the tidiness benefit.
Does macOS use extensions?
Yes, along with additional type information stored with the file. Extensions still determine which application opens a file in most cases, and they matter as soon as you exchange files with anyone on Windows.
What if a file has no extension at all?
The system will not know what to do with it. Identify it from where it came from or from the first few bytes of the file, then add the correct extension. Files arriving from other systems commonly lose theirs.
Which extensions should make me cautious in an attachment?
Anything executable — .exe, .msi, .bat, .cmd, .scr on Windows, .dmg and .pkg on macOS — and archives containing them, along with documents prompting you to enable content or macros. An unexpected attachment of any of these should not be opened.