Front end image path error

I installed a couple of times WP MU, and followed all directions here:
http://codex.ww.wp.xz.cn/Create_A_Network

I installed WPMU for directories (not for subdoman).

On main blog everithing is OK.

On second blog (that I added and called “mag” on /mag subdirectory)
http://www.domain.com/mag/) I have error.

On back end all images are OK, showing on correct path:
/mag/files/2012/01/6402.jpg

But on front end, on posts, images are showing path without /mag/:
/files/2012/01/6402.jpg

What can be a problem?

My .htaccess is

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
</IfModule>
# END WordPress

wp-config.php details:

define('DB_NAME', 'xxxxxxx');

define('DB_USER', 'xxxxxxxr');

define('DB_PASSWORD', 'xxxxxxx');

define('DB_HOST', 'localhost');

define('DB_CHARSET', 'utf8');

define('DB_COLLATE', '');

...

define('WP_DEBUG', false);
define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', false );
$base = '/';
define( 'DOMAIN_CURRENT_SITE', 'mysitename.com' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );