Filter wp_query by post format

filter_post_format.php

<?php

function filter_video_format_posts($query) {
    /* change 999 to the page id for your archive page */
    if ( !is_page ('999') )
        return $query;

    $taxquery = array(
        array(
            'taxonomy' => 'post_format',
            'field'    => 'slug',
            'terms'    => array( 'post-format-video' ),
        )
    );

    $query->set( 'tax_query', $taxquery );

}
add_action( 'pre_get_posts', 'filter_video_format_posts' );