When you attempt to "live" preview an attachment in Outlook 2007, Outlook 2010, or Outlook 2013'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
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.
Note: images (png, jpg, gif) preview automatically. You can disable the image preview handler but cannot set it to Always Ask.
You can disable live preview for any or all attachment types in the Trust Center. Go to Tools, Trust Center, Attachment Handling section in Outlook 2007, or File, Options, Trust Center, Attachment Handling in Outlook 2010/2013.