I've removed Sigil from .jpg and .zip Open With lists, and cleared Sigil's jump list. I'll try to keep an eye on them.
But, given all the .zip files that were in the MRU list were Sigil plugins suggests it's something to do with installing them
Yep, added FootnoteLinker plugin, now it's in the Sigil MRU and .zip Open With has Sigil as an option, and then I added a .jpg via File->Add->Existing files, and it's in the Sigil MRU and Sigil is in the .jpg Open With options.
Well, that wasn't hard.
Fessing up time - I'm using 1.0.0, but adding plugins and images is hardly bleeding edge technology
BR