I assume you are looking at the Folder Options in Control Panel>File Types tab - can you try adding the needed .extension by clicking on New and adding it? I am only guessing really, 'cos I have never needed to do it myself!
What is ransomware and how do I protect my PC from WannaCry?
What I learned from my mentor, Oscar-winning VFX supervisor Phil Tippett
Siri vs Google Assistant
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.