How can I add custom sizes for PDF thumbnails generation?

Question

I define custom image sizes in my theme like this :

add_action( 'after_setup_theme', 'mwm_custom_image_sizes' );
function mwm_custom_image_sizes() {
    add_theme_support( 'post-thumbnails' );
    add_image_size( 'portrait_medium', 200, 286, array( 'center', 'top' ) );
    etc...
}

When I upload an image, the defaut and custom image sizes are created as expected :

But when I upload a PDF file, only the default image sizes for the cover are generated :

How can I tell Wordpress to generate PDF cover images for all image sizes that are defined ?

Answer 1

We have the fallback_intermediate_image_sizes filter within wp_generate_attachment_metadata():

 **
 * Filters the image sizes generated for non-image mime types.
 *
 * @since 4.7.0
 *
 * @param string[] $fallback_sizes An array of image size names.
 * @param array    $metadata       Current attachment metadata.
 */
$fallback_sizes = apply_filters( 'fallback_intermediate_image_sizes', $fallback_sizes, $metadata );

where the default sizes are:

$fallback_sizes = array(
    'thumbnail',
    'medium',
    'large',
);

See core ticket #39231.