Hi @aiko, welcome to the community 
I think you could approach this in a similar fashion to what I have described here:
Essentially convert a single n8n item (your incoming email) with multiple binary files into multiple n8n items each with a single binary file inside the data property.
Your Nextcloud node would then run once for each item it receives and can process a single file with each run. Let me know if you run into any trouble with this 