Add capability to user or role

capability.php

<?php
// Example 1
function add_cap_upload_files_to_user_id() {
  $user = new WP_User( $user_id );
  $user->add_cap( 'upload_files' );
}
add_action( 'admin_init', 'add_cap_upload_files_to_user_id');

// Example 2
function allow_contrib_upload() {

    $contrib = get_role( 'contributor' );

    $contrib->add_cap( 'upload_files' );
}
add_action( 'admin_init', 'allow_contrib_upload', 11 );