How to Detect File Formats — Beyond the Extension

Why File Extensions Cannot Be Trusted

A file named "document.pdf" might actually be:

• A renamed JPEG image
• A Word document with a wrong extension
• A ZIP archive
• Even an executable file

File extensions are just labels — they do not change the actual file content.

How File Detection Works

Every file format has "magic bytes" — specific bytes at the beginning of the file that identify its type:

Format	Magic Bytes	Hex
JPEG	Start with FFD8FF	FF D8 FF
PNG	Start with 89504E47	89 50 4E 47
PDF	Start with %PDF	25 50 44 46
ZIP	Start with PK	50 4B
GIF	Start with GIF89a	47 49 46 38 39 61

Using a File Format Detector

1. Upload any file
2. The tool reads the first bytes of the file
3. It matches against known file signatures
4. Returns the true file type, MIME type, and whether the extension matches

When This Is Useful

• Received a file that will not open — check if the extension is correct
• Security check — verify downloaded files are what they claim to be
• Debugging — determine why a file is not being accepted by a service
• Recovery — identify files that lost their extensions