Your first site on a fresh install will put uploaded files in the traditional location of /wp-content/uploads/, however all subsequent sites on your network will be in the /wp-content/uploads/sites/ folder, in their own subfolder based on the site number, designated by the database. These files will be accessible via that URL.
This is a change from Multisite 3.0-3.4.2, where images of subsites were stored in /wp-content/blogs.dir/ and were shown in https://example.com/files/ and https://example.com/sitename/files and so on. If you started with a Multisite install older than 3.5, it is not an error if your images show with the URL of /files/.
Regardless of WP version, these locations cannot be changed by site admins. Only the network admin can make changes on the site settings page. It is not recommended that you change these without understanding how both the ms-files.php works in conjunction with your .htaccess, as it can easily become non-functional. If the /files/ urls aren’t working, it’s indicative of a misconfigured .htaccess or httpd.conf file on your server.
If you installed WordPress MU in subfolder/subdirectory (not in root folder on your server via ftp) and you have problem with image library, where thumbnails and images do not show, you may need to manually add in rewrite rules for your file directories as follows:
RewriteRule ^([_0-9a-zA-Z-]+/)?siteN/files/(.+) wp-content/blogs.dir/N/files/$2 [L]
Note* modify siteN in the sitename and modify (directory folder upload) N in the (2,3,4 etc).
Put those below the normal call for uploaded files.
If you initially installed WP MU or Multisite MU you will have blog.dir if you initially install WordPress 3.5 or higher wp-content/uploads/sites/ folder (2,3,4 etc).
in network settings there are instructions on how to configure .htacces and wp-config.php
backup your .htaccess file via permalink settings write in a sheet which you are using and try by clicking save.
if it doesn’t work copy your .htaccess code in the forum code tags.
You have direct.access to database (phpmyadmin or similar) ?
