@ toutsursam -
1. In FTP change blogs.dir/#/files to blogs.dir/#/uploads realizing that "#" would be the number of the site.
2. In .htaccess change this line from:
# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]
to
# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?uploads/(.+) wp-includes/ms-files.php?file=$2 [L]
Re-upload .htaccess
3. Go to SuperAdmin - Sites - And Edit - url should be:
wp-admin/ms-sites.php?action=editblog&id=1
4. Look for Fileupload Url and change the word "files" to "uploads"
5. Look for Upload Url Path and change the word "files" to "uploads"
6. Save settings
Hope it works for you - but if not - just remember to backtrack all the steps above and undo your changes!