Thanks Paul...wasn't sure if I was missing anything important. For anyone who is interested, I did find some useful information at the following
links.
Understanding Mime Content Type Header Field Having read these documents and then looking at the properties of an email
that was sent from
Java (that is created via MimeMultipart) I discovered the following (within the properties):
Content-Type: multipart/mixed
...which means that the type of the email message is multipart and the subtype is mixed. Therefore when sending a message of this type, a default must be at work (with the subtype set to mixed).
multipart tells the receiver that the email may contain within the single message a number of seperate data types. Images, video, text documents etc