Add mime types to WordPress

functions.php

/**
 * Add new allowed media types
 */

function mjt_add_mime_types( $wp_get_mime_types ) {
    $wp_get_mime_types['webp'] = 'image/webp';
    $wp_get_mime_types['svg'] = 'image/svg+xml';
    $wp_get_mime_types['docx'] = 'application/vnd.openxmlformats-officedocument.wordprocessingml.document';
    $wp_get_mime_types['pdf'] = 'application/pdf';
    return $wp_get_mime_types;
}

add_filter( 'mime_types', 'mjt_add_mime_types' );