Cannot send emails with attachments

Hi, I’m using ERPNext v12.x.x-develop on Frappe Framework v11.0.3-beta.29.

It seems that I cannot send emails with attachments.
I create a new communication and add any file as an attachment, and then send it. “Communication” is created and the status shows “Send”, but I don’t see any related email in Email Queue List. The recipient does not receive the message either.

I looked through the log files, but I don’t find any suspicious lines.

Do you have any ideas on what is going on, and/or where should I look into?

Kind regards,