1).Assembler component of XMLTransmit pipeline cann't deal with the message formatted as HTML content type. Always use PassThruTransmit pipeline to send out HTML Email messages. All pre-defined pipelines use first message part as a body of message. Thus, you need to define new message in the orchestation of a type that supports custom format (RawString for example) and sent it to PassThruTransmit.
2). In the case you need only attachement - produce new pipeline with MIME/SMIME encoder component and redefine 'Send body part as attachement' on the this component. For InfoPath you need to add XML assembler component to created pipeline. Set its 'Add processing instructions' property to 'Create New' and 'Add processing instruction text' set to the InfoPath specific PI. like
<?mso-infoPathSolution solutionVersion="1.0.0.38" productVersion="11.6250.6250" PIVersion="1.0.0.0" href="http://frameserver/leumi/ApprovalForm.xsn" ?><?mso-application progid="InfoPath.Document"?>
3). In the case you need both HTML formatting and attachement, you need to define multi-part message in the orchestration.