Wordpress - adding Featured image to custom Post Type

Probably this would help

    function create_post_type() {
  register_post_type( 'sadaf_films',
    array(
      'labels' => array(
        'name' => __( 'Films' ),
        'singular_name' => __( 'Film' )
      ),
      'public' => true,
      'has_archive' => true,
      'supports' => array( 'title', 'editor', 'custom-fields','thumbnail' ),
    )
  );
}
add_action( 'init', 'create_post_type' );

This might help someone,

add_theme_support('post-thumbnails');
add_post_type_support( 'my_product', 'thumbnail' );    
function create_post_type() {
        register_post_type( 'my_product',
            array(
                'labels' => array(
                    'name' => __( 'Products' ),
                    'singular_name' => __( 'Product' )
                ),
                'public' => true,
                'has_archive' => true
            )
        );
    }
    add_action( 'init', 'create_post_type' );

$property  = new Cuztom_Post_Type( 'Property', array(
    'supports' => array('title', 'editor', 'thumbnail')
));

I appear to have solved my own question - see above

You can simply enable support Post thumbnail for any custom post type with the following line of code in the theme's function.php file.

add_post_type_support( 'forum', 'thumbnail' );

Note: Here, the forum is the post type name.

You can keep this code in the after_setup_theme hook.