"Content-Type: application/x-unknown " , different response in IE 4.0 & 5.0

To make the browser ask "Save File to Disk" , I make "Content-Type: application/x-unknown " and download the file from the remote location successfully. This works perfectly fine in IE 5.0 and NN 4.0 onwards but in IE 4.0 , It displays the file to be downloaded instead of asking "Save File to Disk"

Help Please URGENT

Here is the full code
print "Content-Type: application/x-unknown\n";
print "Content-Disposition: attachment; filename=$file_name\n\n";
$output_file = read_file($target_file);
print $output_file;

## Sub read_file
sub read_file {
    local($fname) = @_;
    local($content);

    open(FILE, "<$fname") ||  print "cannot open ".$!;

    while(<FILE>)
    {
        $content .= $_;
    }
    close(FILE);

    return $content;