I have a file with a .kv6 extension. I used Slab6 (the correct and only program) to edit this file. Slab6 was set as the default program to open it upon double click, but today it stopped using that and reset to an unknown file. I tried to set it again and when I hit "Browse" and select slab6 again it acts like it chooses it but doesn't actually show up as the default program or Other Programs. I can choose whatever other program and it will show up as a new Default Program but won't work to open it.
Go to Control Panel\All Control Panel Items\Default Programs click Associate a file type with a program. Find the .kv6 extension, double-click it then select your program. Hopefully that solves your problem.