Non-retina icon for .XLS files in Finder

So, like a lot of Mac users, I love a nice-looking Finder environment. One thing I've noticed is that icons for .XLS (older Excel) files are still an older non-Retina version.

I have Excel 16.16.8, which has in its Contents/Resources folder a very nice Retina icon for .XLS files, but the Finder doesn't seem to be using that.

Up until recently, I also had a copy of Office 2011 still on the hard drive, but that's been removed. I tried clearing the icon cache, to no avail.

First, I wonder if this is something that all Excel users have going on -- admittedly, it's not that common to use .XLS files anymore, so maybe people haven't noticed? At any rate, if it's an inherent problem and not something with my computer, I'd like to know so I can stop troubleshooting.

Next, I'm wondering if there's any way you can determine where the Finder is getting its icon for any particular file. Something in the command line maybe?