custom admin thumbnail for videos, not getting post's id?

Question

In the attachments edit page i'm trying to use the selected thumbnail instead of the default icon. 99% of the way there but i can't get the attachment's id into the function.

Am i missing something obvious? Help appreciated! Dc.

function set_attachments_icon($icon, $mime = null, $post_id){
    $vars = get_defined_vars();
    print_r($vars);

    echo " ??".$icon."?? ".$post->id.' '.$post_id.'  ?? id not getting picked up?';

    $thumb_id = get_post_meta($post->ID,'dsv_thumbnail_id', true);

    if($thumb_id!=''){
        $image_attributes = wp_get_attachment_image_src( $thumb_id );
        $icon =  $image_attributes[0];
    }

    return $icon;
}

add_filter('wp_mime_type_icon', 'set_attachments_icon');
apply_filters( 'wp_mime_type_icon', $icon, $mime, $post_id );

Answer 1

First, you don't need the apply_filters below. That is called directly from the wp_mime_type_icon() function already.

The issue is that your add_filter() call is missing the $accepted_args parameter. The add_filter() function has the following construct:

add_filter($tag, $function_to_add, $priority = 10, $accepted_args = 1)

So the $accepted_args is 1 by default. Because of this, only the first argument of $icon gets passed into your function. You need to use the following code when adding your filter:

add_filter('wp_mime_type_icon', 'set_attachments_icon', 10, 3);

Answer 2

The apply_filters call is as follows:

apply_filters( 'wp_mime_type_icon', $icon, $mime, $post_id ); // Last arg is 0 if function pass mime type.

It looks like this is passing the post_id directly and not the post object. That sure would do it! Try this code instead (note that I didn't try it, we're just shooting from the hip here)

function set_attachments_icon($icon, $mime, $post_id){
    if (!$post_id) return $icon;
    $thumb_id = get_post_meta($post_id,'dsv_thumbnail_id', true);
    if($thumb_id){
        $image_attributes = wp_get_attachment_image_src( $thumb_id );
        $icon = $image_attributes[0];
    }
    return $icon;
}
add_filter('wp_mime_type_icon', 'set_attachments_icon', 10, 3);

For me, however, this didn't get hit in the chain because of image_downsize in wp-includes/media.php line 607. You can intercept this to do what you need to do too.

function set_attachments_downsize($return, $attachment_id, $size){
    if ( $size == 'thumbnail' && $thumb_id = get_post_meta($attachment_id,'dsv_thumbnail_id', true) ) {
        $image_attributes = wp_get_attachment_image_src( $thumb_id );
        return $image_attributes[0];
    }
    return $return;
}
add_filter('image_downsize', 'set_attachments_downsize', 10, 3);

Cheers~