I would just like to confirm that a user should never be able to add attachments to a form , InfoPath form or in the out of the box form in SharePoint without having the edit permission?
There was a post I found on this from some years ago and that seems to be the case
Nothing has changed in SharePoint since then? , client is using latest SharePoint online.
I'm having a strange issue where users are seeing very strange behavior when adding attachments, sometimes they can add 1 but not 2 or 2 but not 3 etc , and they are in a permission group that does not have edit permission! just the add permission..
The activity is all done through an InfoPath form in SharePoint.
So I can only assume they must be in a group somewhere on the site that does have edit permission but that doesn't explain the erratic behavior, I thought it was all or nothing?