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File identifier

Drop any file - we read its first bytes locally and tell you the real format, even if it was renamed.

Free tool
File identifierFREE
Drop a file here or browse Nothing is uploaded - checked on your device

How the identifier works

Almost every file format begins with a short, fixed run of bytes called a magic number. A PNG always starts with 89 50 4E 47, a PDF with %PDF, and any ZIP-based file (that includes DOCX and APK) with PK. The name on the file is just a label you can change; the bytes at the front are what the format really is.

Drop a file here and the tool reads only its first 64 bytes, then checks them against a table of known signatures. It all happens in your browser through the FileReader API, so the file never goes anywhere. If the signature and the file name disagree, it tells you the file was most likely renamed.

Good to know

  • It reads the first bytes only, so a 4 GB video is identified as fast as a text file.
  • Nothing is uploaded, so it is fine for private or work documents.
  • A .jpg that is really a .png gets caught, because the bytes give it away.
  • Plain formats like raw .txt or .csv have no signature, so they can come back as "unknown".

Frequently asked questions

Does this upload my file anywhere?
No. The file is read in your browser and its first bytes are checked against a built-in list. Nothing is sent to a server.
It says "unknown signature". What does that mean?
The file does not start with any signature we know. That is normal for plain text like TXT, CSV or JSON, which have no fixed header, and for rare formats. You still get the guess based on the name.
Can it tell if a file was renamed?
Yes. If the bytes point to a different format than the extension, it flags the mismatch, for example a file called photo.pdf that is really a JPEG.
Which formats can it detect?
Around 45 common families: images, audio, video, archives, documents, fonts and programs, including container formats like MP4 and HEIC and the RIFF family (WAV, AVI, WEBP).
Is it safe for confidential files?
Yes. Since the file never leaves your device, there is nothing to intercept or store. Once the page has loaded you can even use it offline.