What Is This File? Identifier
Find out what a file really is, whatever its name says.
How to use the What Is This File? Identifier
- Drop one or more files onto the panel, or click to choose them.
- Each file gets a card with its real type, MIME type and usual extension.
- Read the verdict: it warns when the name and the contents disagree.
- If the extension is wrong or missing, save a correctly named copy.
About this tool
A file's name is only a label, and labels can be wrong. Downloads arrive without an extension, email attachments get renamed, and old backups are full of files called "data" or "export". Programs decide what a file is from its first few bytes instead, because almost every format starts with a fixed signature, sometimes called a magic number. PNG images start with the letters PNG, PDFs with %PDF, and ZIP archives with PK.
This identifier reads those first bytes and recognises around ninety formats: images, camera raw files, office documents old and new, e-books, archives, audio, video, fonts, databases and programs for Windows, macOS, Linux and Android. It looks inside ZIP-based formats to tell a Word file from an EPUB or an Android app, and shows the bytes it used as proof, highlighted in a hex view.
It also acts as a quick safety check. A program disguised as a document, such as a Windows executable named invoice.pdf, or a name like photo.jpg.exe, is flagged in red. When you receive an unexpected attachment, checking it here first costs a few seconds and nothing leaves your computer.
Frequently asked questions
Does it read my whole file?
No. Only the first 64 KB is read, which is enough for every signature the tool knows. Even a file of many gigabytes is identified instantly.
Can it tell me if a file is a virus?
No tool can do that from the format alone. It can tell you the file is a program, and warn when a program pretends to be something else, which is a common trick. Use your antivirus for a full scan.
Why does it say a .docx file is a ZIP archive?
Modern Office files are ZIP archives of XML parts. The tool looks for Word, Excel and PowerPoint folders inside; if a file was renamed or damaged so those are missing, it can only report a plain ZIP.
Why is my file listed as unknown?
Some formats have no signature at all, such as raw sensor dumps, and encrypted files deliberately look random. Very rare or proprietary formats may also not be in the list.
Are my files uploaded for checking?
No. The bytes are read by your browser from your own disk. Nothing is sent anywhere, so confidential documents are safe to check.