If KDE4 mimetypes aren't working, type "kbuildsycoca4" instead of "kbuildsycoca". I believe this change allows for KDE3 and KDE4 apps to coexist. I thought I'd share this since it took me an embarrassingly long amount of time to figure it out :)
I also recall someone saying that the "shared-mime-info" package had to be a certain version. So I found a Fedora RPM for 0.17 and upgraded to that (I know that RPM is old but it was the only one I could find quickly). Not sure if it was required though.