# What Type of File Is This?

A file with no extension, a “.pdf” that won't open, or an attachment that looks suspicious: check what it really is from the first bytes of its contents, and get a copy with the right extension.

_Source: https://sharelargefilesfree.com/tools/file-type-checker · Updated 2026-09-29_

**Quick answer:** This file type checker identifies a file from its signature (the first bytes of its content) rather than its name: images, videos, audio, documents, Office files, archives, fonts and programs, more than 80 formats. It flags wrong or missing extensions and programs disguised as documents, and can return copies with the correct extension. Only the start of each file is read, on your device.

## How to use it

1. Choose or drop the files you want to check.
2. Tick “copies with the right extension” if you want fixed files back.
3. Press Check file types.
4. Read the result for each file, or download the report.

## Why the extension can be wrong

An extension is just part of the name, and anyone or any app can change it. Downloads sometimes lose it, phones save HEIC photos as “.jpg”, and messaging apps rename files. The content, however, starts with a signature (the “magic number”) that says what the file is: `%PDF` for a PDF, `PK` for ZIP-based formats such as Word and Excel, `ftyp` boxes for MP4, MOV, HEIC and AVIF. This tool reads that signature.

## What it recognises

- **Images:** JPEG, PNG, GIF, WebP, HEIC, AVIF, TIFF and camera RAW, BMP, ICO, PSD, SVG, with the size in pixels.
- **Video and audio:** MP4, MOV, MKV, WebM, AVI, MPEG-TS, 3GP, WMV, MP3, M4A, WAV, FLAC, Ogg, AIFF.
- **Documents:** PDF (with its version, and whether it is password-protected), Word, Excel and PowerPoint (old and new), OpenDocument, EPUB, RTF, HTML, XML, JSON, CSV, subtitles.
- **Archives and programs:** ZIP, RAR, 7z, GZIP, TAR, ISO, APK, JAR, Windows EXE, macOS and Linux programs.

## Spot disguised programs

A common trick in scam emails is a program named like a document, such as “invoice.pdf.exe” or an EXE renamed to “.pdf”. If a file is really a program, the checker says so clearly. Don't open it unless you trust the sender. For files you received with a published checksum, also verify them with the [file hash checker](https://sharelargefilesfree.com/tools/file-hash-checker).

## Instant and private

Only the first 64 KB of each file is read (plus the index at the end of ZIP files), so even a 50 GB video is checked instantly. The files never leave your device.

## Frequently asked questions

### How do I find out what type a file is?

Add it and press Check file types. The result shows the real format, its MIME type and whether the extension matches.

### My file has no extension. What do I do?

Check it here, tick “copies with the right extension”, and download the copy: it opens in the right app.

### Can it tell a DOCX from a ZIP?

Yes. It looks inside ZIP-based files and recognises Word, Excel, PowerPoint, EPUB, APK and JAR.

### Is it safe to check a suspicious file?

Yes. The file is only read, never opened or run, and it stays on your device.

### Are my files uploaded?

No. Only the start of each file is read, in your browser.