File Signature Checker — Identify Unknown Files by Magic Bytes Online Free
Identify unknown files by magic bytes. 46+ formats — images, archives, documents, fonts, audio, video, binaries. 100% private — your files never leave your device.
Drop any file here
Identify file type by magic bytes. 46+ formats supported.
Every file format has a signature — a short sequence of bytes at a known offset that identifies the format independently of the file name. JPEG files start with FF D8 FF. PNGs start with \u0089 PNG. PDFs start with %PDF. ZIPs start with PK. These magic bytes are the fundamental unit of file identification — they are what the Unix file command reads, what forensic tools check, and what operating systems use when a file extension is missing or wrong. The File Signature Checker reads these bytes from any file you drop and matches them against a database of 46+ signatures covering images, archives, documents, audio, video, fonts, and executable formats.
The tool is designed for the moment when you encounter a file with no extension, a file with a misleading extension, or a file you downloaded from a source you don’t fully trust. The first 4 bytes tell you more than the file name ever could. A .jpg file starting with %PDF is a PDF with a wrong extension. A video.mp4 starting with PK is a ZIP archive (or a DOCX, or an EPUB). The signature disambiguates these cases instantly.
The privacy model: the FileReader API reads only the bytes needed for signature matching — typically the first 4 to 16 bytes, occasionally up to 512 bytes for signatures with non-zero offsets (like TAR, where the signature is at byte 257). The rest of the file is never read and never buffered. No HTTP request is made after the page loads. Your file stays entirely on your device, and its contents are never seen by any server.
How to use
Drop your file
Drag and drop any file onto the upload area. The tool reads only the first few bytes — the 'magic bytes' or 'file signature' — and matches them against a database of 46+ known file formats. The file content beyond the signature is not read.
Read the results
The output shows the file size, the first 4 bytes in hex, and the detected file type. If multiple signatures match (which can happen when a format reuses another format's magic bytes), all matches are listed — use context to determine the correct one.
Copy or download
Copy the signature report as text or download it. The report is a compact, human-readable summary suitable for pasting into a ticket, an email, or a documentation note.
Frequently asked
What formats are detected?
46+ formats across seven categories. Images: JPEG, PNG, GIF, WebP, BMP, TIFF, PSD, JPEG 2000, JPEG XL. Archives: ZIP, RAR v4/v5, 7z, GZip, BZip2, XZ, TAR (POSIX and GNU), DEB, RPM. Documents: PDF, XML, OLE/DOC. Audio: MP3 (ID3 and MPEG), WAV, OGG, FLAC. Video: MP4, MKV/WebM, MPEG. Fonts: TTF, OTF, WOFF, WOFF2. Binaries: Windows EXE/DLL, Linux ELF, macOS Mach-O, WebAssembly. Other: Shell script, SSL certificate, vCard.
What if the file type is unknown?
If no signature in the database matches, the tool displays the first 16 bytes in hexadecimal. You can look up these bytes manually in a file signature reference like Gary Kessler's File Signatures Table, or use context clues — file name, source, expected content — to identify the format.
Is my file uploaded?
No. The tool reads only the first few hundred bytes of the file using the FileReader API. The rest of the file is never read, never buffered, and never transmitted. The file stays entirely on your device.
Does a matching signature guarantee the file is valid?
No. A matching signature confirms the file header is correct, but does not guarantee the entire file is well-formed. A file could have a valid PNG signature (`PNG`) but a corrupted data section — the signature checker wouldn't detect that. For content validation, open the file in an appropriate application.
How does this differ from MIME type lookup?
The MIME Type Lookup tool maps file extensions to MIME types using a static table. The File Signature Checker reads the actual bytes of the file and identifies the format regardless of what the file extension says. A file named `report.pdf` that starts with `PNG` is actually a PNG image, and the signature checker will correctly identify it as such.
Limitations
- Signature only — no content validationThe checker confirms the magic bytes match a known format. It does not validate the entire file structure, check for corruption, or guarantee that the file can be opened successfully by an application.
- No container introspectionContainer formats like ZIP hold other formats inside them. A `.docx` file is a ZIP of XML files — the checker identifies it as ZIP, not as DOCX specifically. Similarly, EPUB, JAR, APK, and IPA files all share the ZIP signature at the top level.
- No heuristics for text filesPlain text files (`.txt`, `.csv`, `.json`, `.md`, `.html`) have no magic bytes — they are identified by their content, not a fixed header. The checker will report these as unknown.
Platform notes
- macOS
- The Unix `file` command (`file unknown.bin`) provides the same information using the system's magic file database. The browser tool is useful when you don't have a terminal open or when you prefer a visual interface.
- Windows
- Windows identifies files primarily by extension, not content. A file with the wrong extension will be misidentified by Explorer. Use the browser tool to detect the actual format before renaming.
- Linux
- `file` and `xxd` are the standard CLI tools. `xxd -l 16 file.bin` shows the first 16 bytes in hex, which you can compare against magic byte references. The browser tool combines both in one view.
- Web
- Runs entirely in the browser. File data is read locally by the FileReader API — only the first few bytes are accessed, and nothing is transmitted to any server.