What is the difference between a file format and a file extension?

A file format is the actual structure of the data inside the file. A file extension is the few characters after the dot in the filename — a label, and nothing more.

The crucial point is that the extension is not enforced. It is part of the name, chosen by whoever created or renamed the file, and changing it does not change a single byte of content. Renaming photo.jpg to photo.png produces a file that is still a JPEG with a misleading name — and most software will either fail to open it or quietly detect the real format and ignore the extension.

How software actually identifies formats:

Magic numbers. Most formats begin with a distinctive byte sequence — a file signature. PNG files start with a specific eight-byte header; PDFs start with %PDF; ZIP archives with PK. Reading the first few bytes identifies the format reliably regardless of the name.

MIME types — labels such as image/png or application/pdf — are used to declare content type when transferring files over networks.

Structural parsing, where a program attempts to read the file and fails if it does not conform.

Why extensions still matter. They are how the operating system decides which application opens a file by default, and they are how users understand what something is at a glance. Windows in particular relies on them heavily, and hides them by default — which has security consequences.

The security implications are real:

Double extensions. A file named invoice.pdf.exe displays as invoice.pdf when extensions are hidden, and is an executable. This remains a standard malware technique.

Deceptive Unicode characters can reverse the apparent text direction, disguising an extension.

Turn on "show file extensions" — it is one of the cheapest security improvements available.

Containers complicate it further. .mkv and .mp4 are containers holding streams in various codecs, so the extension says little about whether a player can handle the contents.