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What kind of file is this?

Drop any file, even one with no extension or the wrong one. Its first bytes say what it really is, and if the name is wrong you get a correctly named copy.

Bytes of your files sent anywhere: 0 B

How to use it

  1. 01

    Add your files

    Drop one file or a whole batch. Nothing is opened or run, and only the first and last 64 KB of each are read.

  2. 02

    Check the settings

    Each file is matched against more than 100 signatures: the bytes every format starts or ends with.

  3. 03

    Save the result

    See what each file really is and how we know. If an extension is wrong or missing, save a correctly named copy.

Why it doesn’t upload

The files people need to identify are often the ones they shouldn’t send anywhere: a strange attachment, a document with no name, a backup from an old phone. Reading a signature takes a few bytes and no server, so nothing leaves your device.

How to check for yourself

Worth knowing

  • Plain text has no signature. Notes, code, logs and CSV look alike, so the answer for text files is a best reading of the content, and says so.
  • Containers hide their contents. A ZIP can be a Word file, an ebook or a game mod. We look inside for the usual clues, and when there are none we say it is a ZIP and leave the extension alone.
  • It says what a file is, not whether it’s safe. Programs, scripts and shortcuts are flagged, especially ones disguised as documents, but nothing here scans for viruses.

How a file’s type is really decided

The extension is a label on the outside. The format is decided by what the bytes inside are, and nearly every format announces itself in its first few bytes. A JPEG begins FF D8 FF, a PNG 89 50 4E 47, a ZIP 50 4B 03 04 (“PK”, for Phil Katz, who designed it). Word, Excel and PowerPoint files since 2007 are ZIPs too, told apart by the folders inside them.

Operating systems mostly trust the extension, because reading every file would be slow. That’s why a HEIC photo renamed to .jpg shows a JPEG icon and then fails to open, and why “invoice.pdf.exe” is dangerous: Windows hides the “.exe”, so the name says document while the contents are a program.

When the extension is wrong

If the bytes and the name disagree, you get a copy with the right extension: the same bytes under a new name, so nothing about the file changes except that apps now recognise it. If you actually need a different format, the result links to the converter for it.

Some formats legitimately use other names. Camera raw files like DNG and NEF are TIFF inside, comic books (.cbz) are ZIPs, and most Illustrator files are PDFs. Where that applies, the result says the extension is fine rather than “fixing” it.

Questions

How can I tell what type a file is without opening it?

Almost every format begins with a fixed signature: JPEG with the bytes FF D8 FF, PNG with “‰PNG”, PDF with “%PDF-”, ZIP with “PK”. Drop the file here and those bytes are read and matched, without the file being opened or run.

Why won’t my .jpg open?

Usually because it isn’t a JPG. iPhone photos are often HEIC, and images saved from websites are often WebP or AVIF, but they keep a .jpg name when they’re renamed or passed through some apps. Identify the file, save the correctly named copy, then convert it with HEIC to JPG or the image converter if you need a real JPG.

Does changing a file’s extension change the file?

No. Renaming photo.png to photo.jpg changes only the name, not a single byte inside. That’s why a renamed file often won’t open: the app expects one format and finds another. To actually change the format you need a converter.

What is a file with no extension?

It could be anything. Downloads, email attachments and files copied off phones sometimes lose their extension. The contents are fine; the computer just doesn’t know which app to use. Identify it here and save a copy with the right extension.

Is a file called “invoice.pdf.exe” safe?

No. A double extension like that is a classic trick: Windows hides the last part, so you see “invoice.pdf” and double-click a program. This tool flags programs, scripts and shortcuts, and warns loudly when one is named like a document.

What is winmail.dat?

An Outlook wrapper (TNEF) that sometimes arrives instead of the real attachments. The files are inside it; a TNEF or winmail.dat viewer unpacks them, or ask the sender to resend with the message in plain text or HTML.

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