Repository: yt-dlp/yt-dlp
File: yt_dlp/postprocessor/embedthumbnail.py (Python)

Code before:
        convertor.fixup_webp(info, idx)

        original_thumbnail = thumbnail_filename = info['thumbnails'][idx]['filepath']

        # Convert unsupported thumbnail formats to PNG (see #25687, #25717)
        # Original behavior was to convert to JPG, but since JPG is a lossy
        # format, there will be some additional data loss.
        # PNG, on the other hand, is lossless.
        thumbnail_ext = os.path.splitext(thumbnail_filename)[1][1:]
        if thumbnail_ext not in ('jpg', 'jpeg', 'png'):
            thumbnail_filename = convertor.convert_thumbnail(thumbnail_filename, 'png')
            thumbnail_ext = 'png'

        mtime = os.stat(encodeFilename(filename)).st_mtime

        success = True
        if info['ext'] == 'mp3':
            options = [
                '-c', 'copy', '-map', '0:0', '-map', '1:0', '-write_id3v1', '1', '-id3v2_version', '3',
                '-metadata:s:v', 'title="Album cover"', '-metadata:s:v', 'comment="Cover (front)"']

            self._report_run('ffmpeg', filename)
            self.run_ffmpeg_multiple_files([filename, thumbnail_filename], temp_filename, options)

        elif info['ext'] in ['mkv', 'mka']:
            options = list(self.stream_copy_opts())

            mimetype = 'image/%s' % ('png' if thumbnail_ext == 'png' else 'jpeg')
            old_stream, new_stream = self.get_stream_number(
                filename, ('tags', 'mimetype'), mimetype)
            if old_stream is not None:
                options.extend(['-map', '-0:%d' % old_stream])
                new_stream -= 1
            options.extend([
                '-attach', thumbnail_filename,
                '-metadata:s:%d' % new_stream, 'mimetype=%s' % mimetype,
                '-metadata:s:%d' % new_stream, 'filename=cover.%s' % thumbnail_ext])

            self._report_run('ffmpeg', filename)
            self.run_ffmpeg(filename, temp_filename, options)

        elif info['ext'] in ['m4a', 'mp4', 'mov']:
            prefer_atomicparsley = 'embed-thumbnail-atomicparsley' in self.get_param('compat_opts', [])
            # Method 1: Use mutagen
            if not mutagen or prefer_atomicparsley:
                success = False
            else:
                try:
                    self._report_run('mutagen', filename)
                    meta = MP4(filename)
                    # NOTE: the 'covr' atom is a non-standard MPEG-4 atom,

Review comment (suggestion):
re-thinking this, it could also be simplified to
```suggestion
            mimetype = f'image/{thumbnail_ext.replace("jpg", "jpeg")}'
```

Code after:
        convertor.fixup_webp(info, idx)

        original_thumbnail = thumbnail_filename = info['thumbnails'][idx]['filepath']

        thumbnail_ext = os.path.splitext(thumbnail_filename)[1][1:]
        # Convert unsupported thumbnail formats (see #25687, #25717)
        # PNG is preferred since JPEG is lossy
        if info['ext'] not in ('mkv', 'mka') and thumbnail_ext not in ('jpg', 'jpeg', 'png'):
            thumbnail_filename = convertor.convert_thumbnail(thumbnail_filename, 'png')
            thumbnail_ext = 'png'

        mtime = os.stat(encodeFilename(filename)).st_mtime

        success = True
        if info['ext'] == 'mp3':
            options = [
                '-c', 'copy', '-map', '0:0', '-map', '1:0', '-write_id3v1', '1', '-id3v2_version', '3',
                '-metadata:s:v', 'title="Album cover"', '-metadata:s:v', 'comment="Cover (front)"']

            self._report_run('ffmpeg', filename)
            self.run_ffmpeg_multiple_files([filename, thumbnail_filename], temp_filename, options)

        elif info['ext'] in ['mkv', 'mka']:
            options = list(self.stream_copy_opts())

            mimetype = 'image/%s' % ('jpeg' if thumbnail_ext in ('jpg', 'jpeg') else thumbnail_ext)
            old_stream, new_stream = self.get_stream_number(
                filename, ('tags', 'mimetype'), mimetype)
            if old_stream is not None:
                options.extend(['-map', '-0:%d' % old_stream])
                new_stream -= 1
            options.extend([
                '-attach', thumbnail_filename,
                '-metadata:s:%d' % new_stream, 'mimetype=%s' % mimetype,
                '-metadata:s:%d' % new_stream, 'filename=cover.%s' % thumbnail_ext])

            self._report_run('ffmpeg', filename)
            self.run_ffmpeg(filename, temp_filename, options)

        elif info['ext'] in ['m4a', 'mp4', 'mov']:
            prefer_atomicparsley = 'embed-thumbnail-atomicparsley' in self.get_param('compat_opts', [])
            # Method 1: Use mutagen
            if not mutagen or prefer_atomicparsley:
                success = False
            else:
                try:
                    self._report_run('mutagen', filename)
                    meta = MP4(filename)
                    # NOTE: the 'covr' atom is a non-standard MPEG-4 atom,
                    # Apple iTunes 'M4A' files include the 'moov.udta.meta.ilst' atom.
                    f = {'jpeg': MP4Cover.FORMAT_JPEG, 'png': MP4Cover.FORMAT_PNG}[imghdr.what(thumbnail_filename)]