multipart/mixed
- multipart/alternative
  - text/plain
  - text/html
- application/pdf or whatever you want

##</code><code>##

use strict;
use warnings;
use MIME::Lite;

my $alternative = MIME::Lite->new(
    Type => 'multipart/alternative',
);

$alternative->attach(
    Type => 'TEXT',
    Data => 'Message text',
);

$alternative->attach(
    Type => 'text/html',
    Data => '<p><b>Message text</b></p>',
);

my $message = MIME::Lite->new(
    Type => 'multipart/mixed',
    From => 'sender@example.com',
    To   => 'rcpt@example.com',
    Subject => 'Example',
);

$message->attach($alternative);

$message->attach(
    Type => 'TEXT',
    Filename => 'passwd',
    Disposition => 'attachment',
    Path => '/etc/passwd',
);

$message->print(\*STDOUT);