Shared upload folder in wordpress multisite

This will force uploads for all sites to the wp-content/uploads directory. Sub-directories (like year/month) will still exist (if the setting is enabled).

/**
 * Force all network uploads to reside in "wp-content/uploads", and by-pass
 * "files" URL rewrite for site-specific directories.
 * 
 * @link    http://wordpress.stackexchange.com/q/147750/1685
 * 
 * @param   array   $dirs
 * @return  array
 */
function wpse_147750_upload_dir( $dirs ) {
    $dirs['baseurl'] = network_site_url( '/wp-content/uploads' );
    $dirs['basedir'] = ABSPATH . 'wp-content/uploads';
    $dirs['path'] = $dirs['basedir'] . $dirs['subdir'];
    $dirs['url'] = $dirs['baseurl'] . $dirs['subdir'];

    return $dirs;
}

add_filter( 'upload_dir', 'wpse_147750_upload_dir' );