It maybe an upstream bug.
At the bottom of this page
https://github.com/lxqt/lxqt/issues/35
there were issue with mimetypes Oct. 7, 2018. Some posters had improvment by adding
/devel/xdg-utils.
The version of pcmanfm-qt (13.0) in ports was from September 2018
/x11/lxqt/pcmanfm-qt
It looks to have been fixed in pcmanfm-qt 14.0.