How to Check a File Type Before Opening It

A filename extension is useful, but it is not absolute proof of a file's real contents. Someone can rename a file without converting it.

The ending such as .jpg, .pdf or .zip provides the first clue.

Applications and servers may associate files with MIME types such as image/jpeg or application/pdf.

Many formats start with recognizable binary patterns commonly called magic bytes or file signatures. These can provide stronger evidence of the underlying format.

A detector can combine multiple signals instead of trusting the filename alone.

If a file comes from an untrusted source, use appropriate security tools and avoid executing unknown programs. A file-type detector is not an antivirus replacement.

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