Save segments to separate files

[yayayayayayayayayayayaya]

Hi everyone, thanks for this amazing project first of all!

I'm trying to load an mp4 file from disk and saving the audio and video segments back to disk using node & mp4box.js. Now the problem is the segments can't be played using MSE (other segmented mp4 work perfectly, so the issue has to be with my segmentation code). I'm doing the following:

const fs = require('fs')
var MP4Box = require('mp4box')

// ...

var mp4 = MP4Box.createFile()
mp4.onError = function(e) { console.log(e) }
mp4.onReady = function(info) {
    for (var i = 0; i < info.tracks.length; i++) {
        var nbrSamples = Math.round((info.tracks[i].nb_samples / (info.tracks[i].movie_duration / info.tracks[i].movie_timescale)) * 5)
        // ...
        mp4.setSegmentOptions(info.tracks[i].id, null, { nbSamples: nbrSamples, rapAlignement: true })
    }
    const initSegs = mp4.initializeSegmentation()
    for (let i = 0; i < initSegs.length; i++) { 
        // ... (segmentType is either audio or video)
        var path = './media/converted_mp4/TestvideoiOS_' + segmentType + '_init.mp4'
        fs.writeFileSync(path, Buffer.from(new Uint8Array(initSegs[i].buffer)))
    }
    mp4.start()
}

mp4.onSegment = function(id, user, buffer, sampleNum, is_last) {
    // ... (segmentType is either audio or video, segmentCount is segment number)
    var path = './media/converted_mp4/TestvideoiOS_' + segmentType + '_' + segmentCount + '.m4s'
    fs.writeFileSync(path, Buffer.from(new Uint8Array(buffer)))
}

var fb = fs.readFileSync('./media/converted_mp4/original.mp4').buffer
fb.fileStart = 0
mp4.appendBuffer(fb)
mp4.flush()

Is there something wrong with this approach? I can't seem to get these files playing using MSE. I hope somebody can point me in the right direction, because I can't seem to figure out what's going wrong here. Thanks a lot and best regards, Ya