Would it be possible for the MHT MIMEpart order to be changed such that the
html part is always placed at the head of the MHT file?
It appears that Opera expects the first MIMEpart to be the html page
component. If it is html, Opera then processes the rest of the MHT for other
page components (i.e. images and other embedded content). If it isn't html
then Opera displays the first MIMEpart only, and ignores the rest of the MHT
MIMEparts.

On the subject of MHTs, it would probably make sense if there was an MHT
option added to the "Local Structure Type" pull-down menu on the Build tab.