How to use custom icon for desktop submenu?

[Grant Edwards]

How do you specify a custom icon for an XFCE 4.12 desktop menu's
submenu?

For an application rather than a submenu, you can do this in foo.application:

Icon=/home/username/icons/foo.png

But for submenu 'foo' specifying a path for an icon file in
foo.directory (same as above) doesn't work.

AFAICT, for submenus you aren't allowed to specify the path of an icon
file?  You're only allowed to specify an "icon name" or something?

Why?

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