return $ct{'text/plain'} if exists $ct{'text/plain'};
    return $ct{'text/html'} if exists $ct{'text/html'};
    return $parts[0] if $which =~ /text/; 
    return $parts[1] if $which =~ /html/;
[download]

should be

    my $ct = $which eq 'html' ? 'text/html' : 'text/plain';
    return $ct{$ct} if exists $ct{$ct};

    # Fall back to plain text or HTML
    return $ct{'text/plain'} if exists $ct{'text/plain'};
    return $ct{'text/html'}  if exists $ct{'text/html'};

    # Fall back to whatever's first
    return $parts[0];
[download]

By the way,

$ct{$_->content_type} = $_ for @parts;
[download]

should be changed to

$ct{$_->content_type} ||= $_ for @parts;
[download]

to get the *first* part with the right content type, not the last.