Show Featured Image in Archive Post Lists

<?php
add_filter( 'the_excerpt', 'scseed_prepend_thumb_to_excerpt' );
function scseed_prepend_thumb_to_excerpt( $excerpt ) {
    if ( is_singular() || is_admin() || ! has_post_thumbnail() ) {
        return $excerpt;
    }
    $img = get_the_post_thumbnail(
        null,
        'medium',
        array( 'style' => 'max-width:100%;height:auto;display:block;margin-bottom:.75em;' )
    );
    return '<a href="' . esc_url( get_permalink() ) . '" class="scseed-archive-thumb" aria-hidden="true" tabindex="-1">'
        . $img
        . '</a>' . $excerpt;
}