Desktop entry Icons: filename extensions?

[Ville Skyttä]

There seems to be a bit of conflicting (or lacking) information in the
desktop entry and icon theme specs: should one use filename extensions
for Icon entries in .desktop files or not?  Ie. if one installs foo.png
into one of the "correct" icon dirs somewhere, should a .desktop file
intended to use that icon ideally have "Icon=foo.png" or "Icon=foo"?

Desktop entry spec seems to indicate that one should use "foo.png":
http://freedesktop.org/Standards/desktop-entry-spec/0.9.4/ar01s04.html
http://freedesktop.org/Standards/desktop-entry-spec/0.9.4/apa.html

...while the lookup algorithm in the icon theme spec suggests otherwise:
http://freedesktop.org/Standards/icon-theme-spec/icon-theme-spec-0.8.html#icon_lookup

I assume that the filename extension should be included in the value of
the .desktop file's Icon key.  Correct?  If so (or if not), perhaps this
could be clarified in some of the specs?