A file signature (also called magic bytes or magic number) is a unique sequence of bytes at the very beginning of a file that identifies its true format — regardless of the file extension. For example, JPEG files always start with FF D8 FF, PNG files with 89 50 4E 47 (which spells "‰PNG" in ASCII), and GIF files with 47 49 46 38 ("GIF8"). This tool reads those initial bytes directly in your browser and matches them against known signatures to determine the real image format — no upload to any server required.

There are several reasons: security threats (malicious actors may rename executable files with image extensions to bypass upload filters), user mistakes (accidentally saving a PNG as .jpg), legacy systems that don't enforce extension consistency, or intentional obfuscation. By checking the actual byte-level signature, you can detect these mismatches instantly. This is especially important for web applications that accept user-uploaded images — a file named "photo.jpg" might actually be a disguised script or a completely different format.

No. All analysis happens entirely within your browser using the JavaScript FileReader API. The file never leaves your device. Only the first ~100 bytes are read from the file (just enough to identify the signature), so even very large files are processed instantly and privately. For URL checks, a standard HTTP request is made to fetch the image headers, but the full file is never stored or sent to any third-party server.

The tool currently detects: JPEG (FF D8 FF), PNG (89 50 4E 47), GIF (47 49 46 38), WebP (RIFF container with WEBP), BMP (42 4D), TIFF (both little-endian 49 49 2A 00 and big-endian 4D 4D 00 2A), SVG (XML-based, detected by text pattern), ICO (00 00 01 00), AVIF (ISOBMFF ftyp box with avif brand), and HEIC/HEIF (ftyp box with heic/heif/heix brands). Support for additional formats is continuously evaluated.

A file extension (like .jpg, .png) is just a naming convention — it can be changed arbitrarily and doesn't reflect the actual content. A MIME type (like image/jpeg, image/png) is a standardized identifier used by browsers and servers to determine how to handle a file. This tool detects the real MIME type based on the file's internal structure, not its name. Web servers rely on accurate MIME types to serve content correctly; an incorrect MIME type can cause images to fail to display or be mishandled by browsers.

Unlike binary image formats (JPEG, PNG, etc.), SVG is a text-based XML format. It doesn't have a fixed binary magic number. Instead, this tool reads the file as text and checks for the <svg tag or <?xml declaration containing SVG namespace references. SVG files are essentially structured text documents describing vector graphics — which is why they can be opened and edited in text editors, and why their detection requires a different approach from binary formats.