Fix JXL ISOBMFF format

[lpetre]

From the spec: https://github.com/libjxl/libjxl/blob/main/doc/format_overview.md#file-format

An ISOBMFF-based container. This is a box-based container that includes a JPEG XL codestream box (jxlc), and can optionally include other boxes with additional information, such as Exif metadata. In this case, the file starts with the bytes 0x0000000C 4A584C20 0D0A870A.

00 00 00 0C 4A 58 4C 20 0D 0A 87 0A
0  1  2  3  4  5  6  7  8  9  10 11