I’m pretty sure you have to explicitly name all allowed attributes – just use:
$allowedposttags['iframe'] = array (
'align' => true,
'frameborder' => true,
'height' => true,
'width' => true,
'sandbox' => true,
'seamless' => true,
'scrolling' => true,
'srcdoc' => true,
'src' => true,
'class' => true,
'id' => true,
'style' => true,
'border' => true,
);
If you can think of others let me know!