Wordpress Metadata - Update Metadata after Upload

gistfile1.php

/**
 * Add attachment metadata
 *
 * By default, WordPress only adds '_wp_attachment_metadata' metadata to images, audio and video files.
 * This hooks into 'wp_update_attachment_metadata' and sets 'file' to the value of '_wp_attached_file'.
 */
function prefix_update_attachment_metadata( $data, $post_id ) {

    // only set 'file' if needed
    if ( isset( $data['file'] ) ) {
        return $data;
    }

    // get file path e.g. pdf/filename.pdf
    if ( $file = get_post_meta( $post_id, '_wp_attached_file', true ) ) {
      // TODO Do your Metadata stuff here

        $data['file']=$filename."/preview.jpg";
        $data['thumb']=$filename."/preview.jpg";
        $data['mime-type']='image/jpeg';
    }

    return $data;

}
add_filter( 'wp_update_attachment_metadata', 'prefix_update_attachment_metadata', 10, 2 );