...
for my $ja (0 .. $ja_max - 1)
{
    my $attachBinaryFile = $attach_list[$ja];
    my $content_type;

    if    ($attachBinaryFile =~ /\.gif$/i)  { $content_type = 'image/gif' }
    elsif ($attachBinaryFile =~ /\.jpg$/i)  { $content_type = 'image/jpeg' }
    elsif ($attachBinaryFile =~ /\.zip$/i)  { $content_type = 'application/zip' }
    elsif ($attachBinaryFile =~ /\.html$/i) { $content_type = 'text/html' }
    elsif ($attachBinaryFile =~ /\.pdf$/i)  { $content_type = 'application/pdf' }
    elsif ($attachBinaryFile =~ /\.xls$/i)  { $content_type = 'application/vnd.ms-excel' }
    elsif ($attachBinaryFile =~ /\.doc$/i)  { $content_type = 'application/vnd.ms-word' }
    elsif ($attachBinaryFile =~ /\.log$/i)  { $content_type = 'application/octet-stream' }
    else                                    { warn "Unrecognized file type: $attachBinaryFile"; next; }

    $smtp->datasend("--$boundary\n");
    ...