I would suggest you to read the doc of MIME::Lite. But to get your code to work, I often send text/html-attached mails and never had a problem with:

my $msg = new MIME::Lite(
              From    => $defaults{EMAIL_FROM},
              To      => $defaults{EMAIL_TO},
              Subject => $defaults{EMAIL_SUBJECT},
              Type    => 'text/plain'
              );
        $msg->attach(
             Type        => 'text/html',
             Encoding    => '8bit',
             Path     => "$defaults{REPORT_DIR}$defaults{REPORT_FILE_N
+AME}",
                        );
        MIME::Lite->send( 'smtp', 'localhost', Timeout => 20 );

        my ($result) = $msg->send();
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giant