How Image File Signatures Like PNG, JPEG, WebP and AVIF Identify Formats

The article argues that a file's actual byte content is more reliable than its extension or declared MIME type for identifying an image format. Every common image container starts with a fixed byte pattern at a known offset, such as FF D8 FF for JPEG or 89 50 4E 47 for PNG. PNG's signature includes CRLF and DOS end-of-file bytes so that text-mode transfers corrupt it visibly. AVIF, HEIC and HEIF share the same ISO base media container and are told apart only by the brand code at offset 8. The piece also shows a short JavaScript check that identifies common formats from the first twelve decoded bytes, while noting that a signature confirms only the container type, not that the file is complete.