Unable to decode JFXX jpeg

[hey-red]

When I try to identify or decode image that contains JFXX marker, I got an exception System.NotSupportedException: 'Unknown App0 Marker - Expected JFIF.'

I think it's an expected behavior, because jpeg decoder works only with JFIF identifier, but if I just change JFXX to JFIF with hex editor, then image can be sucessfully decoded.

I not sure that this is issue, because it's maybe a MJPEG frame(?), but other tools like image editors, viewers and browsers can display that image,

Example image

[JimBobSquarePants]

Looks like JFXX is a valid JFIF extension APP0 marker.
https://www.w3.org/Graphics/JPEG/jfif3.pdf

Additional APP0 marker segment(s) can optionally be used to specify JFIF extensions. If
used, these segment(s) must immediately follow the JFIF APP0 marker. Decoders should
skip any unsupported JFIF extension segments and continue decoding.
The JFIF extension APP0 marker is identified by a zero terminated string: "JFXX". The
JFIF extension APP0 marker segment contains a 1-byte code which identifies the
extension. This version, version 1.02, has only one extension defined: an extension for
defining thumbnails stored in formats other than 24-bit RGB

JFIF Extension APP0 Marker Segment
Immediately following the JFIF APP0 marker segment may be a JFIF extension APP0
marker. This JFIF extension APP0 marker segment may only be present for JFIF versions
1.02 and above. The syntax of the JFIF extension APP0 marker segment is:

X’FF’, APP0, length, identifier, extension_code, extension_data

        length (2 bytes) Total APP0 field byte count, including the byte
                         count value (2 bytes), but excluding the APP0
                         marker itself
        identifier (5 bytes) = X'4A', X'46', X'58', X'58', X'00'
                              This zero terminated string (“JFXX”) uniquely
                              identifies this APP0 marker. This string shall
                              have zero parity (bit 7=0).

        extension_code (1 byte) = Code which identifies the extension. In this
                                  version, the following extensions are defined:
                                = X'10' Thumbnail coded using JPEG
                                = X'11' Thumbnail stored using 1 byte/pixel
                                = X'13' Thumbnail stored using 3 bytes/pixel

        extension_data (variable) = The specification of the remainder of the JFIF
                                extension APP0 marker segment varies with the
                                extension. See below for a specification of
                                extension_data for each extension

However, the specification seems to say that there should be a JFIF marker before it and other decoders can clearly decode it so we should allow the same. I would say to treat the JFXX marker like the JFIF marker if no JFIF marker is present.