File Formats

It’s the letters after the last dot in a file name, like .pdf or .jpg. It tells your computer which kind of data the file holds and which app should open it.

No. Renaming photo.heic to photo.jpg only changes the label, not the data inside, so most apps will refuse to open it or show an error. Use a converter to change the format.

Windows and macOS hide them by default. On Windows, open File Explorer, choose View, then Show, then File name extensions. On a Mac, open Finder, choose Settings, then Advanced, and turn on Show all filename extensions.

Use the File Info tool on this site. It reads the first bytes of the file, which identify most formats, and tells you if the extension doesn’t match the contents.