This is because the attachments module code was setting the 'object slam' bit by using PermissionMask.All
Solution here is to route the attachment item creation call through the existing inventory code in BasicInventoryAccessModule rather than copy/pasted code in AttachmentsModule itself.
This was happening because we were using the source avatar's item IDs in the clone appearance.
Switch to using the asset IDs of attachments instead for NPCs.
The InventoryAccessModule and AttachmentModule had to be changed to allow rezzing of an object without an associated inventory item.
Hopefully goes some way towards resolving http://opensimulator.org/mantis/view.php?id=5653
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