File extension vs. file format
The extension is the label stuck on the outside of a file. The format is how the data inside is actually arranged. Most of the time they match — but not always, and that gap explains a lot of confusing file problems.
The label and the contents
Think of a jar in the cupboard. The extension is the handwritten sticker on the lid; the format is what's really in the jar. Renaming the sticker doesn't change the jam into honey, and a mislabelled jar will still taste like whatever it contains.
Why one extension can mean several formats
Nobody owns file extensions. Any developer can pick three letters, so the same extension has been reused by unrelated programs across decades. In the National Archives registry, .dbf covers several distinct database formats, and .doc covers both Microsoft Word and older WordPerfect files. Same label, genuinely different formats.
How software really identifies a file
Serious tools ignore the extension and look at the bytes. Most formats start with a distinctive signature — a "magic number" — such as %PDF for PDFs or PK for ZIP-based files. That's how the file command on Mac and Linux identifies things, and how digital archivists assign each format a permanent identifier called a PUID.
What this means for you
- Renaming an extension never converts a file; it only changes which program tries to open it.
- If a file won't open, the extension may be wrong — identify it by content instead.
- To genuinely change format, open the file and use Save As or Export.