Hi Dirk,
Currently Survey123 is not supported to edit the attachement yet and there is a similar existing enhancement request for this:
ENH-000110614 Add the ability to allow additional attachments to be added when attempting to edit and resend surveys in the Survey123 Field App
I encourage you to contact Esri Support. Our Support team will assign an official enhancement number for your records. Similar requests from other customers can then be attached to the same enhancement request, which helps us assess demand for the enhancement and prioritize it accordingly.
Thanks,