Thanks for the reply Khaled.
So it was a bug. Good to know because that would have broken the link we are using in WebMaps if we don't include the ObjectId to it's correct place.
I quess that the Attachment Id's are still FeatureService and not feature wide. So I need to get the attachment id fetched anyway?
Thanks for pointing Upload API out, haven't checked that earlier. In our case, we want to have our information (feature information and the picture) used easily in the ArcGIS platform products. If you compare the usage between Upload and Feature Attachments, do you think that Upload can be considered as easily consumed manner? If I understood the concept of Upload correctly, I still want to use Feature Attachments as a storage for images so those can be easily accessed from WebMaps and other clients. For example, it is easy to attach the image for WebMap's popup with Image tag.
with regards,
Antti