Better handling for atypical streams (random attachment etc.) in Matroska alike

[idest]

1. Create a new video from the files attached to this ticket with the command:

   ffmpeg.exe -i input.mkv -attach text.txt -metadata:s:t mimetype=text/plain -c copy out.mkv
   

2. Examine the video with

   ffprobe.exe out.mkv
   

Notice the "Unsupported codec with id 0 for input stream 1" error/warning at the end of the output.

FFmpeg version: N-118325-gea381285e7-20250119,
OS: Windows 10 Home.

[idest]

FWIW, ffprobe emits similar errors/warnings for attachments of other MIME types, for example, if I create a new video with the command

ffmpeg.exe -i input.mkv -attach DejaVuSans.ttf -metadata:s:1 mimetype=application/x-truetype-font out2.mkv

and then examine it with ffprobe:

ffprobe.exe out2.mkv

I get output that ends with "Unsupported codec with id 98304 for input stream 1".

[MasterQuestionable]

͏    Maybe it should just copy as-is. (for unknown streams)

͏    See also:
͏    ​https://trac.ffmpeg.org/ticket/11200#comment:8