Hi,
There are two methods portals can store attachments
1. Through Dataverse Notes (Annotations) table (guessing your portal stores the attachment using this)
2. If sharepoint integration is enabled and portal has been configured to use sharepoint as the repo of the attachments
If you use method 1, you can try on your power automate to have it trigger on add of notes similar below and do the necessary steps needed on the attachment
Note that the Document column will contain the base64 string of the file and you'll need to have additional checks if an attachment exists on the Note record
Hope this answers your question.
