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 => 'firstname.lastname@example.org',
To => 'email@example.com',
Cc => 'firstname.lastname@example.org, email@example.com',
Subject => 'A message with 2 parts...',
Type => 'multipart/mixed'
### Add parts (each "attach" has same arguments as "new"):
Type => 'TEXT',
Data => "Here's the GIF file you wanted"
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 );