[Exim] Syntax has me stumped

This isn't working:

file = ${extract{5}{:}{${lookup \
{$local_part}lsearch{/etc/vmail/passwd.$domain}{"{$value}.forward"}}}}

What I'm trying to do is a lookup and extract (does work) and append
".forward" onto the value extracted and set file = to that value. I
can't get the command right.