apply:include, currying, free var...then a surprise!

?- apply:include(file_name_extension(_,'.pl'),['a.pl','b.txt','c.pl'],PrologFiles).
PrologFiles = ['a.pl'].

?- [user].
isplf(F) :- file_name_extension(_,'.pl',F).
|: % user://1 compiled 0.07 sec, 2 clauses
true.

?- apply:include(user:isplf,['a.pl','b.txt','c.pl'],PrologFiles).
PrologFiles = ['a.pl', 'c.pl'].

Is this an addressable bug, a problem that exists between keyboard and
chair, or a known problem I should learn to live with?

As an aside, please ignore my not using prolog_file_type/2.

-- 
Ian Tegebo