In the previous post, we explored files and directories. We learned that every piece of data on a computer is stored as a file, and directories act like boxes that organize those files. But if you look more closely at a file’s name, you’ll often notice a period followed by a few letters. This is called a file extension.

In this post, we’ll take a closer look at file extensions.

What Is a File Extension?

A file extension is a short suffix at the end of a filename (after the dot) that identifies the file type and hints which app can open it. For example:

• report.docx → Microsoft Word document
• photo.jpg → Image file
• song.mp3 → Audio file

Think of extensions like tags on a blog post: they label content for both you and the computer, signaling whether something is a document, picture, song, or even a program.

Common Extensions (Quick Glance)

• .txt — plain text
• .jpg — image (photo)
• .mp3 — audio
• .mp4 — video
• .pdf — portable document
• .exe — executable program (Windows)

How Extensions Link Files to Apps

Operating systems associate extensions with default apps—for instance, .docx files open in Word and .jpg in Photos on Windows. You can change these associations at any time (e.g., set .jpg to open in Photoshop by default).

Show Extensions (Helpful Habit)

Windows hides extensions by default, which can be confusing. Enable them so you always see the real type (in Windows 11: File Explorer → View → Show → File name extensions). It’s a small change that makes file management clearer.

Security Tip

Malware sometimes disguises itself with misleading names or icons. Watching the actual extension (e.g., noticing .exe vs. .jpg) helps you avoid running dangerous files. Keeping extensions visible is one of the simplest safety practices.

Summary

• Extensions label what kind of file something is.
• They connect files to the right applications.
• Visible extensions improve safety and efficiency.

Just like .txt means text, .jpg means image, and .mp3 means music, understanding extensions clarifies how computers handle files.

You can view the original Korean blog post at the link below

View in Korean