[postprocessor/embedthumbnail] Allow mkv to embed thumbnails

Fixes #6046

youtube_dl/postprocessor/embedthumbnail.py

@@ -40,7 +40,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
                 'Skipping embedding the thumbnail because the file is missing.')
             return [], info
 
-        if info['ext'] == 'mp3':
+        if info['ext'] in ('mp3', 'mkv'):
             options = [
                 '-c', 'copy', '-map', '0', '-map', '1',
                 '-metadata:s:v', 'title="Album cover"', '-metadata:s:v', 'comment="Cover (Front)"']