It is not possible to add a out-of-the-box field for the Original Filename of the entity file field
User interface changes
The new field label will have to be named with the desired name
If form alter is going to be user, refer to https://api.drupal.org/api/drupal/core!modules!system!form.api.php/funct...
Original report by andypost
Not sure we able to add a "original filename" field for file entity - this cold be a feature but Very useful in success D8MI so probably we need a follow-up here