I'm sorry to say, but there is no way to for you to control the Content-Disposition of an attachment added through APEX_MAIL.ADD_ATTACHMENT. The infrastructure is there in the API to support this, perhaps in a future patch set or APEX version, but I fully realize that this doesn't help you today.
I thought so. I will put this in the Apex feature request database.
BTW, for other people it might be interesting to know that there is another option I tried that doesn't work in many situations either. You can put this in the email body:
where v_image_base64 is the base64 encoded jpg image.
From my experiments the inline attachment with a cid reference seems to work best.