[Exim] expand $n0-$n9 in system filter

Hi.

In system filter place next part:
add 1 to n0
mail text     "mail from $h_subject with param $n0"
     expand file path_to_template

In file path_to_template put:
mail from $h_subject with param $n0

In "text" field exim expanded both variable, in field "expand file" -
only $h_subject. It's bug or feature ?

b.r.
Kozin Maxim