Hi Christian
First of all, the use of Mail Package is deprecated.
Refer to Stefan's blog below on how to create an email with attachments without Mail Package.
Create email with body and attachments for binary payload with Java mapping
However, if you insist on using Mail Package, you need to set the ContentDisposition to "inline" so that the Main Document is set as the mail body. You can use MessageTransformBean to set it - refer section 3.2.4 in the following document
How to Use the Mail Adapter of the Exchange Infrastructure 3.0 J2EE Adapter Engine (NW2004)
Here's another alternative using Mail Package where you manually construct the whole MIME stream for the body and attachments.
How send e-mail with XML + PDF and body text
Rgds
Eng Swee