Filr: Send e-mail before file expires.

functions.php

<?php

add_action( 'filr_before_check_expire_date', function( $expiration_date, $today ) {
    $difference = $expiration_date->diff( $today );

    // Get difference in days.
    if ( $difference->d <= 2 ) {
        // prepare and send the e-mail.
        $to = 'you@company.com';
        $subject = 'File will expire in 2 days.';
        $headers = array( 'Content-Type: text/html; charset=UTF-8' );
        $body = '';

         // Now we can send the e-mail.
        wp_mail( $to, $subject, $body, $headers );
     }
}, 10, 2 );