Returns the file extension appended to the
names of modified files under inplace-edit mode. This value can be set
using ARGF.inplace_mode= or passing the -i switch to the
Ruby binary.
static VALUE
argf_inplace_mode_get(VALUE argf)
{
if (!ARGF.inplace) return Qnil;
return rb_str_new2(ARGF.inplace);
}