prefix_only_permit_zips.php

<?php
// Set the list of permitted mime types to zip only
function prefix_only_permit_zip( $permitted_mimes ) {
  $permitted_mimes = array( 'zip' => "application/zip" );
  return $permitted_mimes;
}
add_filter( 'pewc_permitted_mimes', 'prefix_only_permit_zip' );

// Add PDF to the list of restricted filetypes
function prefix_protected_directory_zip_only( $restricted_filetypes ) {
  $restricted_filetypes = array( 'zip' );
  return $restricted_filetypes;
}
add_filter( 'pewc_protected_directory_allowed_filetypes', 'prefix_protected_directory_zip_only' );