Embedded images in Nextcloud Mail

It seems to me Nextcloud Mail isn’t capable of making the association between the Content ID of an embed image inserted in the HTML by the email client of the sender of the mail and the attachment. I certainly receive email with images but I can’t understand if they use CID, if they embed the images in base64 or if the images are actually loaded from an external URL.
My problem is that the system I’m building to automate some emails in another platform seems to produce email of which the embed images are not correctly displayed by Nextcloud Mail: if I use Content ID the mail just shows the Content ID:

If I convert the image in base64 and insert it directly in the HTML it gets displayed well.