Why do extensions lie?

Extensions are just names anyone can change — malware relies on it (invoice.pdf.exe). Magic bytes describe actual content.

Can two types match at once?

Yes — containers share signatures (DOCX/XLSX/JAR are all ZIPs). All matches are listed so you can see the ambiguity.

Does this scan the whole file?

No, only the header. That identifies type instantly but cannot rule out appended malicious payloads — pair with antivirus.

File Type Validator

Identify a file’s true type from magic bytes and flag mismatched extensions.

Verify true file types by magic bytes instantly. Free file signature checker detecting renamed PNG, JPEG, PDF, ZIP, MP4 and EXE files.

About File Type Check

Identify a file’s true type from magic bytes and flag mismatched extensions.

Select any file to read just its first 32 bytes and match them against known magic numbers — PNG, JPEG, GIF, PDF, ZIP, MP4, executables, and more. The verdict contrasts the detected type with the file extension to catch renames.

Only headers are read, so even huge files check instantly (capped at 25 MB for safety). Polyglot files can match multiple signatures; all hits are listed. Everything stays in your browser.

Where it is used

Spotting renamed executables, validating uploads, forensic triage, and debugging corrupt downloads.

How to use the File Type Check

Select File (up to 25 MB) — select a file (*/*) — Only the first bytes are read. Files never leave your browser..
Calculate — press the Calculate button to see the result instantly above.

Formula

Compare first 32 bytes against a magic-number table; contrast with extension.

Examples

Example

Result: Select a file — e.g. a renamed photo.jpg.exe flags as MZ executable

Frequently Asked Questions

Why do extensions lie?: Extensions are just names anyone can change — malware relies on it (invoice.pdf.exe). Magic bytes describe actual content.
Can two types match at once?: Yes — containers share signatures (DOCX/XLSX/JAR are all ZIPs). All matches are listed so you can see the ambiguity.
Does this scan the whole file?: No, only the header. That identifies type instantly but cannot rule out appended malicious payloads — pair with antivirus.

Calculator Inputs