In bug #3484665 it was suggested to add the categories Qt and KDE. Well, smplayer doesn't use KDE and so that's wrong (the meaning is defined at http://standards.freedesktop.org/menu-spec/latest/apas02.html\)
In any case, as can be seen at http://quickgit.kde.org/index.php?p=kdelibs.git&a=blob&h=9abfd6e3ea4625cb4c4a01dbacc68940f6947b20&hb=8f8737c7cb5c0e46acd850bcb7bd4be76cb5fb0a&f=kio%2Fkio%2Fkrun.cpp, adding the KDE category is equivalent to add the entry
The protocols supported by KIO and smplayer are, to the best of my knowledge, unrelated. So the correct fix is to add the entry X-KDE-Protocols with whatever protocols are really supported (I guess this depends on mplayer, but still...). And the KDE category should be removed.