Choose a file to read its first bytes — its signature or magic numbers — and identify the type those bytes actually indicate.
Every common format starts with fixed bytes called a signature or magic number: a PNG always begins with the same eight bytes, a PDF with %PDF, a ZIP with PK. The identifier reads the first 32 bytes of the chosen file, shows them in hexadecimal, and matches them against a table covering images, documents, archives, audio, video and executables. The file's extension is compared with the detected type and a mismatch is flagged, because an extension is only a label — a renamed file keeps its original signature.
What is a file signature or magic number? It is a short sequence of bytes at the start of a file that identifies its format regardless of its name. Operating systems and tools use it to recognise files whose extension is missing or wrong.
What does a mismatch warning mean? The bytes indicate one format while the extension claims another — for example a file named photo.jpg whose bytes are a PNG. It often means a simple rename, but it is also how disguised files are spotted, so the content should be treated with care.
Which types are recognised? PNG, JPEG, GIF, BMP, WebP, TIFF, PDF, ZIP, 7z, RAR, gzip, MP3, MP4 and MOV, WAV, Windows EXE, Linux ELF, SQLite and tar. Plain text and other formats without a fixed signature report as unrecognised.