What some email client (ipad ... ) chooses to display or not display, how it behaves, there is very little you can do to influence that -- there is no problem to be solved from your end

The stackoverflow thread you linked is different problem from the one you described, but it contains code, even if it is php

Your post contains no code , and your problem statement is that there is nothing wrong with the email, except some ipad email client doesn't hide attachments -- it doesn't have to hide them

There is no problem, the email isn't malformed, the client isn't broken, there is no problem, nothing to be fixed, nothing you can do