Hello,
As per my finding, you have to custom create event receiver to handle this situation. Use "ItemAttachmentDeleting" event to prevent user to delete attachment. Please refer below MSDN link to implement this:
http://stackoverflow.com/questions/2023414/prevent-deletion-of-attachments-from-a-sharepoint-list-item
Hope it could help
Hemendra:Yesterday is just a memory,Tomorrow we may never see
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