Sometimes I think this could be caused by Windows post-7 deliberately awful handling of file associations, which becomes increasingly untransparent and uncustomizable.
A while ago I was having issues with my default browser not opening .maff files (compressed saved webpages), neither by double-clicking local files in file explorer nor by dragging them to the browser window - i'd get that same infamous slashed circle.
As modifying the registry manually is a complex task, I was glad i found
SetUserFTA, which helped me solve this specific case.