the problem with W10 i found is that it takes over the preferred/default program settings etc etc. so, if your preferred image viewer was - say - irfanview - W10 will change that to its own viewer, similarly - if you want irfanview to automatically open image files and you select the image file extensions in the irfanview preferences dialogue - it will accept those - but - it will also tell you that you have to go to the relevant W10 setup (control panel) dialogue to set those prefs in W10. i have even experienced that W10 will change those settings back to their own prefs at some stage (not sure what the conditions are). ie. those W10 prefs are not always as sticky as they should be - apparently.
you have to look for the default program settings dialogue in the Control Panel.
oh - and in case you haven't noticed = each iteration of windows seems to bury the settings deeper and deeper into the control panel structure - i guess so that MS can control your machine and your preferences. that's just plain irritating!
Start -> Settings from the popup gear icon on the left second up nornally -> (should look like below - select the one underlined in green (I think, but you'll be roughly there)
