Automatically assign video ‘poster’ value to ‘filename’ for archive listing

Thanks for your help.
Managed to figure out a temporary solution, should anyone else have a similar query.

Temporarily edited media.php

$filename = basename( wp_get_attachment_url( $attachment_id ) );

    $default_types = wp_get_video_extensions();
    $defaults_atts = array(
        'src'      => '',
        'poster'   => "diary/wp-content/Pictures/posters/".$filename.".jpg'",
        'loop'     => '',
        'autoplay' => '',
        'preload'  => 'none',
        'width'    => 640,
        'height'   => 360,
        'class'    => 'wp-video-shortcode',
    );

I’ve stripped the url from the wp_get_attachment_url() attachment and plopped it directly into the ‘poster’ attribute.

Next, I’ll have to figure out a way of writing it as a function, rather than edit media.php on every update 🙂