silverstripe-framework/javascript/src/UploadField_select.js

import $ from './jQuery';

$.entwine('ss', function($) {
	// Install the directory selection handler
	$('form.uploadfield-form .TreeDropdownField').entwine({
		onmatch: function() {
			this._super();

			var self = this;
			this.bind('change', function() {
				// Display the contents of the folder in the listing field.
				var fileList = self.closest('form').find('.ss-gridfield');
				fileList.setState('ParentID', self.getValue());
				fileList.reload();
			});
		},
		onunmatch: function() {
			this._super();
		}
	});
});