[PeerTube] The downloader can't combine/mux two fragmented MP4 files

Checklist

• I am able to reproduce the bug with the latest version given here: CLICK THIS LINK.
• I made sure that there are no existing issues - open or closed - which I could contribute my information to.
• I have read the FAQ and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected version

0.28.0

Steps to reproduce the bug

See #11601. The downloader is unable to combine/mux two segmented MP4 files and will generate a file with only audio and with strange intermittent silences. ffmpeg can combine the two files without issues. These two PeerTube videos are examples: https://peertube.qontinuum.space/w/k2wKrFgGpoSSrGTCkkjoLC https://video.blast-info.fr/w/87GhAwHborpk5Kym1eqZ6m . To make the download dialog provide the option to download video+audio, change the following in SecondaryStreamHelper:

Turn this:

        } else if (mediaFormat == MediaFormat.MPEG_4) {
            return audioStreams
                    .stream()
                    .filter(audioStream -> audioStream.getFormat() == MediaFormat.M4A)
                    .max(ListHelper.getAudioFormatComparator(MediaFormat.M4A,
                            ListHelper.isLimitingDataUsage(context)))
                    .orElse(null);

into this:

        } else if (mediaFormat == MediaFormat.MPEG_4) {
            return audioStreams
                    .stream()
                    .filter(audioStream -> audioStream.getFormat() == MediaFormat.M4A
                            || audioStream.getFormat() == MediaFormat.MPEG_4)
                    .max(ListHelper.getAudioFormatComparator(MediaFormat.M4A,
                            ListHelper.isLimitingDataUsage(context)))
                    .orElse(null);

It would be good if someone who has some experience with the MP4 format (standard ISO/IEC 14496-12) could work on this.