When you attempt to "live" preview an attachment in Outlook 2007′s reading pane and uncheck the box to "Always warn before previewing", this type of file if you want to restore this setting, you'll need to edit the registry.
Subkeys for each file type is stored in HKEY_CURRENT_USER\Software\Microsoft\Office\Previewers
Delete the key for the file types you don’t want to automatically preview and press F5 to refresh the registry key before returning to Outlook.
More Information
You can disable live preview for any or all attachment types in the Trust Center. Go to Tools, Trust Center, Attachment Handling section.
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Last reviewed on Sep 28, 2011
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