Reg info goes here:
HKCR
  \.ext
    \shellex
      \{CLSID of interface, ie IExtractImage}

Extension approval:
HKLM
  \SOFTWARE
    \Microsoft
      \Windows
        \CurrentVersion
          \Shell Extensions
            \Approved

For debugging extensions:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
  create a DWORD called DesktopProcess with a value of 1