Application Express :: Change Content-disposition In Email Attachment?

I am using apex_mail (in Apex 4.1) to send emails with an attachment.

v_mail_id := apex_mail.send(
p_to => 'somebody@gmail.com'
...etc...
);
apex_mail.add_attachment( p_mail_id => v_mail_id
,p_attachment => v_image
,p_filename => 'signature.jpg'
,p_mime_type => 'image/jpeg');

Apex creates this as an attachment:

Content-Disposition: attachment;

I would like to change it to inline:

Content-Disposition: inline;

so I can reference it in my email body with <img src="cid:signature.jpg"/>. Well... I hope it is going to be shown inline anyway if I change Content-Disposition.

Currently (with Content-Disposition: attachment;) it works for Outlook, but, for instance, not in Gmail in the browser. Gmail shows the image separately as an attachment.

Is there a way to change the content-disposition with Apex?

I know it is possible to do it with utl_mail, but this is currently not installed in our databases. I need to involve our DBA to set this up, and I don't know if he is willing to do that.