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Upgraded from 1.9.15 to the stable 1.10. Upgrade went well, just noticing the following IO Exception in my log quite a bit. "Z" folder is in there, just no attachement with that file name...wasn't in my 1.9.x backup either.
Server 2012 R2 Datacenter (fully patched)
IIS 8 (And yes, the IUSR account IIS uses for anonymous web has FULL access to the 'attachments' folder)