What Is a File Extension and Why It Matters
A file extension is the group of letters after the last dot in a file name, such as .docx or .jpg. It tells the computer which program should open the file. Understanding extensions helps you open files correctly, spot suspicious downloads and save files in the format you need.
Common file extensions
| Extension | Type | Usually opened by |
|---|---|---|
| .docx | Word document | Microsoft Word, Google Docs |
| .xlsx | Excel spreadsheet | Excel, Google Sheets |
| Portable document | Any PDF reader, browsers | |
| .jpg or .png | Pictures | Photo viewers, browsers |
| .mp4 | Video | Video players |
| .zip | Compressed archive | Built-in unzip tools |
Show extensions on Windows
By default Windows may hide known extensions. Open File Explorer, select the View menu, then Show, and turn on File name extensions. This lets you see the real type of each file.
Show extensions on Mac
In Finder, press Command + Shift + Period to show hidden files, or open Finder settings under Advanced and choose to show all filename extensions.
Why extensions matter for safety
Criminals sometimes disguise a dangerous file as a harmless one. A file named "invoice.pdf.exe" is a program, not a PDF, even though the first part looks like a document. Always check the last extension before opening a file from an email or download.
Change an extension correctly
Changing the extension does not convert the file. A photo renamed to .png may not open. Use the program's Export or Save As command to convert a file to a different format.
Frequently asked questions
Can a file have two extensions?
Yes. Archives such as .tar.gz have two layers. The last extension usually describes the outer format.
What if the extension is missing?
Open the file with a program you expect, or ask the sender which program they used. Do not open unknown files from unknown senders.
See also how to spot phishing emails, which uses similar signs.
Extensions for code and data
Developers and analysts meet extensions such as .csv, .json and .txt often. A CSV file is plain text with values separated by commas, and it can be opened in a spreadsheet. A JSON file is structured text used by many apps and web services. Knowing these extensions helps you choose the right program and avoid corrupting data by opening it in the wrong editor.
Keep the extension when you rename
When renaming a file, keep the extension at the end of the name. Removing it can make the system guess the wrong program. If you type a new name without a dot and extension, Windows may warn you, and macOS may ask for confirmation. Always check the extension after a rename.
Related reading
- How to Read Computer Error Messages and What to Do: A method for understanding error messages on Windows and Mac: what to read first, how to search for solutions safely, and when to get help.
- How to Keep Windows Secure with Microsoft Defender: A checklist for Windows security: turning on Microsoft Defender protection, running scans, enabling Firewall and checking account security.
- How to Set Up a New Computer in One Afternoon: A checklist for setting up a new Windows or Mac computer: updates, accounts, backups, security settings, essential apps and moving your files.