On a file...
left mouse click "Opens" it
right mouse click shows the "Menu".
With middle mouse click you could "Show" ("View") the file.
Because you might want to use the alternative application to view the file,
or you might have some unknown files,
and you can view them quickly with xfv,
and see if they are text files or what kind of text files they are.
Middle mouse click could work for "Edit",
but people more often "View" a file, than "Edit" it
(or at least, I do).
Within the "fast" spirit of xfe...