"managed_file" form field type returns null value in my form submission
After alot of digging/debugging i discovered that i could not pull any data from the "manage_file" field while using the PrivateTempStore in my Multi-Step form. After disabling the form cache, i began to get the values and the expected behavior by using: 

$form_state-&gt;disableCache(); 


My Submit Handler   

public function buildForm(array $form, FormStateInterface $form_state) {
 /**
 *  Fixes image upload bug in form. Does not work unless we 
 disable the cache while using PrivateTempStore
 **/

 $form_state-&gt;disableCache();

.......

}


See https://www.drupal.org/project/drupal/issues/2647812#comment-11683961