Work on #1491.

islandora.libraries.yml

@@ -0,0 +1,7 @@
+image_media_form_defaults:
+  version: 1.x
+  js:
+    js/image_media_form_defaults.js: {}
+  dependencies:
+    - core/jquery
+    - core/jquery.once

js/image_media_form_defaults.js

@@ -0,0 +1,8 @@
+(function (Drupal, $) {
+  "use strict";
+  var ImageName = $('input[id=edit-name-0-value]').val();
+  $(document).ajaxSuccess(function() {
+    $('input[data-drupal-selector=edit-field-media-image-0-alt]').val(ImageName);
+  });
+}) (Drupal, jQuery);
+