File Field description field default using tokens?
Good news - it's not that hard!  Here's what you need to do:


Install Drupal Console (if you haven't already): https://drupalconsole.com/
Install the devel module: https://www.drupal.org/project/devel
Enable the webprofiler module (part of devel) drush en webprofiler
Webprofiler adds a toolbar to the bottom of your browser window.  Use this to get the form ID of the form you want to modify.
Use Drupal Console to generate a hook_form_FORM_ID_alter() function.  You'll need either a custom theme or a custom module for this.


Warning: If you're modifying the node edit form, it probably uses the admin theme.  You probably don't have a custom admin theme, so you may want to put this in a custom module.

Customize your function to prepopulate the field the way you want.  Here's what I did:

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter() on behalf of coit_customizations.module.
 *
 * Provides a default for file description on meeting edit form
 */
function coit_customizations_form_node_meeting_edit_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  // Check to see if there's already a file description
  $existing_description = $form['field_supporting_documents']['widget'][0]['#default_value']['description'];

  // If there isn't, add the default
  $new_description = $existing_description ? $existing_description : 'Download Supporting Documents';
  $form['field_supporting_documents']['widget'][0]['#default_value']['description'] = $new_description;
}

Don't forget to enable your module!


For further reading:


http://enzolutions.com/articles/2016/02/01/form-altering-will-not-be-the-same-in-drupal-8-after-using-webprofiler/
How to change a field&#39;s description using hook_form_alter in Drupal 8?
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.4.x
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_FORM_ID_alter/8.4.x


One last tip: xdebug is very useful for inspecting your form array so you know what to customize.  If you're trying to do this kind of thing and aren't using xdebug, I recommend you check it out.  

As an easier alternative, just add this to your function: drupal_set_message($form);