Attachments are currently not supported by the Email module.
If you want this, you could create a new feature request in the MXDN for this.
The API does support attachments, so if you create on the email entity a reference set towards system.filedocument you can call in java:
EmailModule.mail(config, htmlCode, PlainMessage, Subject, ToList, CCList, BCCList,context, __Attachments);
As you can see this also allows html email. The only trick here is that you have to modify the html code for images on send to replace mendix guids
with off course filename the proper file name and also you have to attach the image files.