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Put thie code in the function.php file of your template.

post_type is the name yout your post type 🙂

An important information is, the SIZE of the image returned in should be exist in your thumbs.

function custom_admin_thumb_size( $thumb_size, $thumbnail_id, $post ) {

if( 'download' === $post->post_type ) {
return array( 300, 200 );
}

return $thumb_size;
}
add_filter( 'admin_post_thumbnail_size', 'custom_admin_thumb_size', 10, 3);