• Go to Outlook
• Click File tab, then Options and finally Advanced
• Now scroll down to “Display” and check the “Disable hardware graphics acceleration” option

According to a post by “David at DRA” this is due to a graphics driver bug that has been introduced during a Win 10 update. So hopefully this is a temporary issue that will be resolved.  (See half way down this page http://answers.microsoft.com/en-us/office/forum/office_2016-outlook/outlook-2016-attachments-will-not-open-in/ce844af4-1013-4e47-bd1c-573ae7124667?page=2)

More Detail:

Not sure of exact timing, but somewhere in the process of upgrading to Windows 10, Office 2016 and/or installing updates the ability to open Office documents directly from Outlook stopped working.

The errors related to “Protected View”, with the following error message.

The vast majority of fixes suggesting having to save the file to disk and then right clicking properties and checking the “Unblock” field:

However thanks to a post by David at DRA I found the fix outlined at the top.  This is not ideal and Microsoft hopefully have this on their list of “bugs to fix”. Until then this has stopped the pain.