Perhaps you should try using a content type of multipart/mixed.

The following example is an excerpt from MIME::Lite.

Create a multipart message (i.e., one with attachments) and send it SMTP

    ### Create a new multipart message:
    $msg = MIME::Lite->new(
        From    => 'me@myhost.com',
        To      => 'you@yourhost.com',
        Cc      => 'some@other.com, some@more.com',
        Subject => 'A message with 2 parts...',
        Type    => 'multipart/mixed'
    );

    ### Add parts (each "attach" has same arguments as "new"):
    $msg->attach(
        Type     => 'TEXT',
        Data     => "Here's the GIF file you wanted"
    );
    $msg->attach(
        Type     => 'image/gif',
        Path     => 'aaa000123.gif',
        Filename => 'logo.gif',
        Disposition => 'attachment'
    );
    ### use Net:SMTP to do the sending
    $msg->send('smtp','some.host', Debug=>1 );
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