Xfce does not have a concept of a default File Manager. Currently in some places Thunar is just hardcoded, and sometimes you can change it. A related issue is that the MIME handler for folders is never set, which leads to problems with applications that use MIME to query the right File Manager. The solution comes down to adding a new helper that can be chosen from the Preferred Applications dialog. And perhaps the MIME entry could be updated automatically? Not sure about the latter..
Created attachment 1865 Implement a new helper This patch implements a new helper FileManager.
Created attachment 1866 Add filemanagers to choose This patch adds Thunar, PCMan File Manager and Nautilus.
Created attachment 1878 Fixed patch including exo-open change Apparently my patch was incomplete, particularly I forgot to include exo-open/ exo-url to actually use the file manager preference. I double checked and actually added a missing --help string for exo-open (the new FileManager type) and a missing error message if no file manager is set.
As for the MIME handling I think Preferred Applications should also update the folder handler implicitly. Otherwise users might end up with the wrong file manager if something relies on the MIME database rather than exo. See http://bugzilla.gnome.org/show_bug.cgi?id=418760 In the Gnome bug report Thunar actually is 'by accident' at the top of the MIME list for folders and thus used in Gnome.
Slightly modified patch applied because of some code changes. Thanks.