File Type Detector — Identify Files by Magic Bytes

Identify any file from its contents

A file name's extension is only a label, and labels can be wrong. Downloads lose their extension, email systems rename attachments, and files copied from phones or old backups often end up called file or IMG_0001 with no hint of what they contain. This detector ignores the name and looks at the bytes instead. Almost every binary format starts with a distinctive signature, called magic bytes: 89 50 4E 47 for PNG, FF D8 FF for JPEG, %PDF- for PDF, PK for ZIP and 37 7A BC AF 27 1C for 7-Zip. The tool compares your file with well over a hundred signatures covering images, audio, video, archives, documents, fonts, executables, databases and disk images.

Some formats need a closer look. MP4, MOV, M4A, HEIC and AVIF all share the ISO base media structure, so the detector reads the brand in the ftyp box. DOCX, XLSX, PPTX, EPUB, ODT, JAR and APK are all ZIP files, so it inspects the names of the first entries. Old Office documents share the OLE2 container, and text files are recognised as JSON, XML, HTML, SVG, PEM keys, vCards or plain UTF-8 and UTF-16 text.

Spot mismatched and suspicious extensions

For every file you get the detected format, its standard MIME type, the category and the extensions that format normally uses, next to the type your browser guessed from the name. When the extension does not match the contents you see a warning. Usually this is harmless — a PNG saved as .jpg still opens — but an “invoice.pdf” that turns out to be a Windows executable is a common way to spread malware. The hex dump of the first 64 bytes, with offsets and an ASCII column, lets developers verify the result themselves.

Only the first 64 KB of each file is read, and everything happens inside your browser, so even very large videos or disk images are checked instantly and nothing is uploaded. You can check many files at once and copy a short text report for each one.