Why Older Systems Misidentify AVIF as MP4 Files

Older operating systems often misidentify AVIF (AV1 Image File Format) images as MP4 video files because both formats rely on the exact same underlying architecture: the ISO Base Media File Format (ISOBMFF). When legacy software encounters an AVIF file, it inspects the internal container structure rather than the compressed image data. Lacking the modern definitions required to interpret AVIF image headers, the operating system falls back on its existing database of media signatures, which maps that specific container format directly to MP4 digital video.

The Shared ISOBMFF Container

At a structural level, AVIF is not an independent image format like JPEG or PNG. Instead, it is an image profile built on top of the High Efficiency Image File Format (HEIF), which in turn derives from the standardized ISO Base Media File Format (ISO/IEC 14496-12). This container organizes data into hierarchical blocks known as "boxes" or "atoms."

Because the MP4 video container is also an implementation of ISOBMFF, an AVIF file and an MP4 video file share identical top-level binary structures. Both begin with a File Type Box (ftyp), followed by metadata and media data payloads structured in the same byte layout.

Magic Byte Sniffing and the ftyp Box

When an operating system attempts to identify an unknown file type—or verify a file whose extension may be missing or unassociated—it reads the initial bytes of the file, known as "magic numbers" or file signatures.

In both AVIF and MP4 files, the file begins with the ftyp atom at byte offset 4. Inside this box are declarations for the "major brand" and "compatible brands." An AVIF file uses brands such as avif or avis. However, older systems designed before these brands were registered do not recognize the avif token. Instead, their file-type sniffing routines simply detect an ISOBMFF container starting with an ftyp box. Because MP4 was historically the predominant format using this signature, the legacy OS defaults to labeling the container as an MP4 video.

Lack of Native Codec Integration

Operating system components such as Windows Explorer, macOS Finder, or older Linux desktop environments rely on installed media splitters and decoders to generate thumbnails and determine MIME types. On older platforms:

Without modern codec libraries to clarify that the container houses a single AV1-compressed still image rather than an H.264 or H.265 video stream, the system assigns the file an MP4 MIME type (video/mp4) and may attempt to open it with default media players rather than image viewers.