Detect a GIF file before choosing its browser preview

A GIF file may actually contain MP4 video data, impacting upload widgets. A brief header examination can provide an early error message before the application passes the bytes to a decoder. The following JavaScript function inspects the two GIF signatures and a common ISO Base Media File Format header. It does not assume the file is a playable MP4.

The six-byte GIF marker is sufficient for this classification. However, this does not confirm every block is valid or that decoding will fit the device's memory. A header check is a routing guide, not an acceptance check. Other formats using the same container family should not be misidentified as MP4. The filename and content type are separate evidences; the upload endpoint should validate the format independently.

If the header identifies an ISO BMFF, inform the user that it looks like a video container. If the kind is unknown, clarify that the file doesn't have a recognized GIF header. Otherwise, indicate a GIF header was found. Avoid assuming renaming will fix the file; changing the extension does not alter the encoding. Similarly, tools labeling looping animations as GIFs are incorrect.

Apply this distinction to saved animations. Some services, like Twitter's GIF Downloader, demonstrate converting MP4 to GIF for X animations. If a destination only accepts images, convert the file to GIF and inspect it. If the destination supports video and playback controls are useful, retain the MP4 format. The browser should only display a GIF preview if the image decoder accepts it.

A video container requires a supported video codec and container combination; the header alone cannot determine this. Server-side policies should enforce file size, decoded dimensions, resource limits, and a supported-format policy.