I created a function which gets the metadata video length from the selected attachment. I use this together with a plugin called multisite-global-media which lets me share media from a site to all other sites in the multisite network. But selecting a video from this media site will break my function, as I am not able to pull the metadata from attached media from the other site except if I use the switch_to_blog function.

But I need to determine if the selected media is from the current site or from the media site.

This is what I am working on:

switch_to_blog($attachment_site);

$attachment_id = attachment_url_to_postid( $_POST[ 'background-video' ] );
$video_meta = get_post_meta( $attachment_id , '_wp_attachment_metadata', true ); 

restore_current_blog();

if( isset( $_POST[ 'background-video' ] ) && $video_meta['length'] >= 1 ) {
    update_post_meta( $post_id, 'slide_duration', $video_meta['length'] );
}

What I need to do is to get the $attachment_site somehow.

Any idea how?

EDIT:

if( isset( $_POST[ 'background-video' ] ) ) {
    update_post_meta( $post_id, 'background-video', $_POST[ 'background-video' ] );
}