I take your point but sigil is an epub processor and an epub with a strange file type is still really an epub, inside....
is there not something inside a file that tells windows what it "really" is & could not sigil not look at that, rather than at the filetype shell.
& yes I think it should open zips if the zip is really an epub. I am surprised it does not already do zip.
is it not just a case of removing the line of code that inspects file type & says "quit already" ? & let sigil at least try to open it