"Open With" Command Missing from the Context Menu

When you double-click an unknown file type, an unexpected default
action may occur.

When you right-click an unknown file type, the Open With command may
be missing from the context menu that appears.

Use Registry Editor to examine the file association key for unknown
file types:

HKEY_CLASSES_ROOT\Unknown\shell\openas\command
The "default" value for this key should read:

C:\%windir%\rundll32.exe shell32.dll,OpenAs_RunDLL %1