Detect a GIF file before choosing its browser preview

A file called reaction.gif can still contain MP4 video. This matters in upload widgets: choosing an <img> solely from the filename can produce a broken preview, while accepting the file as a verified image can mislead later processing. A short header check can improve the error message before the application hands the bytes to a real decoder. Read a small prefix, not the whole file The following function checks the two GIF signatures and a common ISO Base Media File Format header. Its iso-bmff result deliberately does not claim that the file is a playable MP4. Other formats use the same container family. async function inspectHeader(file) { const bytes = new Uint8Array(await file.slice(0, 32).arrayBuffer()); const ascii = (start, length) => String.fromCharCode(...bytes.slice(start, start + length)); if (bytes.length >= 6 && ['GIF87a', 'GIF89a'].includes(ascii(0, 6))) { return { kind: 'gif', mimeHint: 'image/gif' }; } if (bytes