not include desktopdb.mk.  You can check if its gnumeric.desktop file
  has a MimeType entry and, if so, include desktopdb.mk in the Makefile,
  rebuild and reinstall gnumeric.  If it works, please file a PR :)

gnumeric does not have a MimeType entry.  I was able to, on my
machine, associate gnumeric with excel's mime type in nautilus, and
then opening .xls worked.
I hadn't read the desktop spec before, so probably didn't give
'gnumeric %F' properly.