In attachment tables, the download/synchronization/upload of attachments to fail. The relationship between the FeatureClass and Attachment table can be via FC ObjectID or FC GlobalID.
When creating an editable FeatureClass that can contain attachments, create it in this order:1. Create the feature class.2. Enable attachments on the feature class.3. Insert GlobalIDs into the feature class.This will work because the relationship between the feature class and the attachment table will be FC ObjectID based...as it was originally.