还原选项“始终在预览此类型文件前发出警告”（Restore the "Always warn before previewing" dialog）

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.

如果你在outlook中预览附件时，不小心去除了默认勾选的“始终在预览此类型文件前发出警告”，以后再也没有提示了，要恢复以前的预览方式，只有编辑注册表，outlook中没有对应的此选项，

删除以下路径中的子键HKEY_CURRENT_USER\Software\Microsoft\Office\Previewers

如删除DOCX后再预览word文档时将会发出警告。