# Check what a file really is, not what its name says

> Drop files to see their real type from their first bytes, and whether the name fits. Nothing is uploaded.

Reads the first bytes of any file to find its real type. Flags a program hidden behind a photo or document name, and names that do not fit. Free.

- Tool: [File type checker](https://fixmint.cc/file-type-checker)
- Category: [Files & archives](https://fixmint.cc/files)
- Fixes: Is this file what its name says?
- Runs in the browser: files and text stay on the device, no sign-up, free.

## How to check what type a file really is

1. Drop one or more files on the mat. Nothing is uploaded.
2. Read the real type next to the name it has.
3. Do not open a file marked Do not open until you know where it came from.

## Questions

### How does it know the type?

Most formats begin with a fixed run of bytes. A PNG starts with 89 50 4E 47, a PDF with %PDF. The checker reads the first 4 KB of each file and compares them with that list. It does not trust the name.

### Why is a double extension dangerous?

Windows hides known extensions by default, so photo.jpg.exe looks like a photo. A file whose last part is a program type is flagged even when the first part looks harmless.

### Can it prove a file is safe?

No. It tells you what the file is and whether its name fits it. A matching name does not make the content harmless, and an unknown type is not proof of danger.

## Related tools

- [ZIP viewer](https://fixmint.cc/zip-viewer): What is inside this ZIP?
- [Check file corruption](https://fixmint.cc/check-file-corruption): Is this file broken or cut off?
- [Compare binary files](https://fixmint.cc/compare-binary-files): Are these two files the same?