example.php

add_action( 'after_setup_theme', 'thumbnail_meta' );

function thumbnail_meta(){
  Thumbnail_Meta::create( [
            'html_thumbnail_meta_id'          => [
              'label' => __( '<strong>Featured Settings:</strong>' ),
              'input' => 'html'
            ],
            'checkbox_thumbnail_meta_id'        => [
              'label' => __( 'Checkbox?' ),
              'input' => 'checkbox'
            ],
            'url_thumbnail_meta_id'      => [
              'label' => __( 'Link:' ),
              'type'  => 'url',
            ],
            'select_thumbnail_meta_id' => [
              'label'   => __( 'Display' ),
              'input'   => 'select',
              'options' => [
                'none'   => '-- None --',
                'top'    => 'Content Top',
                'right'  => 'Content Right',
                'left'   => 'Content Left',
                'bottom' => 'Content bottom',
                'lhs'    => 'LHS'
              ]
            ]
          ] );
  }

Thumbnail_Meta.php

<?php
/**
 * Thumbnail Meta
 */

class Thumbnail_Meta
{
    //Holds Media Fields
    private $media_fields = [];

    //Default arguments.
    private static $default_item_args = [
        'application' => 'image',
        'exclusions'  => ['audio', 'video'],
        'input'       => 'text'
    ];

    /**
     * Factory method for syntactical suger.
     * @param $args
     */
    public static function factory( $args ){
    	new Thumbnail_Meta( $args );
    }

    /**
     * Thumbnail_Meta constructor.
     * @param $args
     */
    function __construct( $args )
    {

        $this->media_fields = $args;

        add_filter( 'attachment_fields_to_edit', [ $this, 'render' ], 9, 2 );
	    add_filter( 'attachment_fields_to_save', [ $this, 'save' ], 9, 2 );

    }
    
    function

    /**
     * Render
     *
     * @description             Method responsible for generating fields in the Media Editor.
     *
     * @param $form_fields
     * @param $post
     * @return mixed
     *
     * @since                   2.32.0
     */
    public function render($form_fields, $post = null)
    {

        // We browse our set of inputs
        foreach ( $this->media_fields as $field => $values )
        {
            $values = wp_parse_args( $values, self::$default_item_args );

            // If the field matches the current attachment mime type
            // and is not one of the exclusions
            if(
                false !== strpos( $post->post_mime_type, 'image' )  &&
                ! in_array( $post->post_mime_type, $values['exclusions'] ) )
            {
                // We get the already saved field meta value
                $meta = get_post_meta( $post->ID, "_" . $field, true );

                // Define the input type to "text" by default
                switch ($values['input'])
                {
                    default:
                    case 'text':
                        $values['input'] = "text";
                        break;
                    case 'url':
                        $values['input'] = "text";
                        break;
                    case 'textarea':
                        $values['input'] = "textarea";
                        break;
                    case 'select':

                        // Select type doesn't exist, so we will create the html manually
                        // For this, we have to set the input type to "html"
                        $values['input'] = "html";

                        // Create the select element with the right name (matches the one that wordpress creates for custom fields)
                        $html = '<select name="attachments[' . $post->ID . '][' . $field . ']">';

                        // If options array is passed
                        if(isset($values['options']))
                        {
                            // Browse and add the options
                            foreach ($values['options'] as $k => $v)
                            {
                                // Set the option selected or not
                                if($meta == $k)
                                    $selected = " selected='selected'";
                                else
                                    $selected = "";

                                $html .= '<option' . $selected . ' value="'.$k.'">' . $v . '</option>';
                            }
                        }

                        $html .= "</select>";

                        // Set the html content
                        $values['html'] = $html;

                        break;
                    case 'checkbox':

                        // Checkbox type doesn't exist either
                        $values['input'] = "html";

                        // Set the checkbox checked or not
                        if( $meta )
                            $checked = ' checked="checked" value="1"';
                        else
                            $checked = ' ';

                        $html = '<input' . $checked . ' type="checkbox" name="attachments[' . $post->ID . '][' . $field . ']" id="attachments-' . $post->ID . '-' . $field . '" />';

                        $values['html'] = $html;

                        break;
                    case 'html':

                        $values['input'] = "html";
                        $html = "";

                        if( isset( $values['html'] ) )
                        {
                            $html .= $values['html'];
                        }else{
                            $html .= '<span></span>';
                        }

                        $values['html'] = $html;

                        break;
                }

                // And set it to the field before building it
                $values['value'] = $meta;

                // We add our field into the $form_fields array
                $form_fields[$field] = $values;
            }
        }

        // We return the completed $form_fields array
        return $form_fields;
    }

    /**
     * Save Meta
     *
     * @description             Handle data changes.
     *
     * @param $post
     * @param $attachment
     * @return mixed
     */
    function save($post, $attachment )
    {
        foreach ( $this->media_fields as $field => $values) {

            $values = wp_parse_args( $values, self::$default_item_args );

            if( $values['input'] === 'checkbox' ){
                if (isset($attachment[$field])) {
                    update_post_meta( $post['ID'], "_" . $field, 1);
                } else {
                    update_post_meta( $post['ID'], "_" . $field, 0);
                }
            } else {
                if (isset($attachment[$field])) {
                    update_post_meta( $post['ID'], "_" . $field, $attachment[ $field ] );
                } else {
                    delete_post_meta( $post['ID'], $field);
                }
            }
        }

        return $post;
    }
}