When will I see a MIME attachment in my send items?

When checking the sent items, you will see only a few emails with a mime-header, or an extra tab called Message Source

When you send an email that should generate a MIME attachment (e.g. HTML email with attachment) in general there will no MIME attachment be generated for outgoing messages. The GWIA is responsible for this, and will only provide an MIME attachment to the external recipients. There is 1 exception. When the email contains an embedded image, the client will always generate a MIME attachment.

This is working as designed