.htaccess SSL – wp-content files are not HTTPS

I’m running WP in multisite mode.
I secured my server with a SSL Certificate and added the lines :

define(‘FORCE_SSL_LOGIN’, true);
define(‘FORCE_SSL_ADMIN’, true);
in the wp-config.php.

After that I added following lines to the .htaccess:

# BEGIN WordPress

RewriteEngine On
# Redirect all to https
RewriteCond %{SERVER_PORT} 80
RewriteCond %{REQUEST_URI} /
RewriteRule ^(.*)$ https://nurb.de/$1 [R,L]

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]

# END WordPress

So the complete site is working with HTTPS including the Admin.

But I still have the pictures coming from HTTP. If you inspect the HTML Site, I found that the images are loaded by only HTTP not HTTPS.
So some browsers are not willing to show them, because some stuff is HTTPS and some is HTTP.

How do I have to change the .htaccess file to get the wp-content files (all files) accessed via HTTPS and solve this problem!?????????

Or what do I have to change to solve it, if it’s not the .htaccess file????

King regards

Guido