DTOO110 - Blocking as default file block opening behavior must be enforced.
Users can open, view, or edit a large number of file types in Office 2013. Some file types are safer than others, as some could allow malicious code to become active on user computers or the network. For this reason, disabling or not configuring this setting could allow malicious code to become active on user computers or the network.
Set the policy value for User Configuration -> Administrative Templates -> Microsoft Excel 2013 -> Excel Options -> Security -> Trust Center -> File Block Settings 'Set default file block behavior' to 'Enabled: Blocked files are not opened'.