[DiapDealer]

But there HAS to be some sort of Windows setting that affects what show up in the list. How else to explain that after opening html files, using Add Existing to add jpg|png|gif files, adding plugins (zip files), I still don't see anything BUT .epubs in the Windows MRU list for Sigil?

I would be more than happy to say it's expected Windows behavior except for one nagging thing: it doesn't happen on any of my systems (3 Win10 and 1 Win7). I'm trying my damndest to get ANYTHING other than epub show up in the list, and I can't! I'm using Sigil to try and open every file-type under the sun and the only things that get added are epubs I've recently opened.

And why does adding a jpeg file to an epub in Sigil add Sigil to the Windows Open With list for jpgs for BetterRed, when it does nothing of the sort for me? Same for adding plugins (.zips)?

Something obvious has to being missed here! Otherwise the different behaviors being experienced just don't make sense.