Change default file manager for kde

You can also do this in a GUI via kcontrol, but I am originally running gnome with fluxbox so I had to find out another way to do this, since it affects several apps like ktorrent and transmissions actions when chosing to open a folder via them.

It was actually pretty easy:
1.) Open a terminal and edit your ~/.local/share/applications/mimeapps.list
2.) Go to /usr/share/applications and check out what your filemanagers folder handler file is called. E.g. thunars is Thunar-folder-handler.desktop, which I will use (case sensitive).
3.) In the mimeapps.list you opened, edit or add the following line:
inode/directory=(your-file-handler);
Example:
inode/directory=Thunar-folder-handler.desktop;

Save, and relogin to your session ;) It shall now work!

PS1: The file mimeapps.list may not exist. If it doesn't create it. I had to do that one.
PS2: You can also try to edit defaults.list if located in the same directory and add the line there.