Overview:
Invaild context menu entry("Set as wallpaper") on specific desktop items.

Additional Information:
Actually I'm not sure if this is thunar's bug or xfdesktop's:
thunar 1.6.3, build flags: https://projects.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/thunar&id=96bd64517954a88733a04b75f8281ec9290621ca#n30
xfdesktop 4.10.2, build flags: https://projects.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/xfdesktop&id=154f7ee567f66307cf6f5c98037266cb98817a81#n24

Steps to Reproduce:
1.Create a new empty file on desktop.
2.Right click the new empty file(or, the text file) on desktop.

Actual Results:
There is an invaild menu entry "Set as wallpaper" on the context-menu of a normal-text-file(i.e. empty file).

Expected Results:
Menu entry "Set as wallpaper" should not appear on a text-file's right-click menu.

Temporary Workaround:
Recompile thunar with '--disable-wallpaper-plugin' is a workaround, but this also disable the 'Set as wallpaper' entirely.