Related to this ticket: http://sourceforge.net/tracker/?func=detail&aid=3186318&group_id=156956&atid=801867
I was looking a little for a list of configuration options but I couldn't found any. My next idea was to look in the libfm/pcmanfm configuration files in /etc/xdg and ~/.config if there is a default entry for this option. The default pcmanfm.conf in my home folder isn't even complete (the volume part is missing). Maybe there are another global or local options which are missing there.