Syncing audio recordings. Is there a way?

Occasionally I make mp3 audio recording on my android which I want to keep on the main server. It is a part of my ‘creative approach’.
I can use the auto upload quite well for images but now I am setting up a custom folder, pointing to my audio recordings, for my audio files and Nextcloud doesn’t see them at all, so it doesn’t transfer them. It is as if the only file types it can see are images.
Has anyone managed to autoupload .mp3 files successfully?
Thanks in advance!

OK. Situation resolved. Don’t panic!!
It seems that the custom audio file folder auto-upload process takes quite a bit longer than a photo upload.
Instead of 20 to 40 seconds it takes about 5 minutes. This might be due to filesize or time-outs during transfer, maybe something else too, but its there now!
I still need to determine if there is a feasible maximum audio duration. It might need a tweak to the config.php to facilitate the extra transfer duration.
Any hints welcome!