fix(admin): refine .mjs related NC28 upgrade info

To cover rewrites, Cache-Control header, and give a concrete config file for MIME types, as suggested here: #11471 (comment)

Signed-off-by: MichaIng <[email protected]>

admin_manual/release_notes/upgrade_to_28.rst

@@ -12,9 +12,11 @@ Web server configuration
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 * The recommended :ref:`nginx configuration<nginx-config>` changed as Nextcloud Talk now serves audio files with ``.ogg`` / ``.flac`` extension, make sure to add these extensions to the list of static files.
-* As some core app now make use of JavaScript modules, make sure your web server is serving ``.mjs`` files with JavaScript MIME type.
-  When using Apache with ``.htaccess`` configuration this will be done automatically, if your Apache is not configured to use the ``.htaccess`` files make sure ``.mjs`` is added as a file extension for JavaScript in ``conf/mime.types``.
-  For NGinx please refer to the our recommended :ref:`nginx configuration<nginx-config>`.
+* As some core app now make use of JavaScript modules, make sure your web server is not rewriting requests to ``.mjs`` files, but serves them with `text/javascript` MIME type and proper `Cache-Control` header, like ``.js`` and other static file extensions.
+
+  * When using Apache with ``.htaccess`` configuration, this will be done automatically.
+  * For Nginx, please refer to our recommended :ref:`Nginx configuration<nginx-config>`.
+  * For other setups, make sure to add ``.mjs`` to the list of static file extensions in web server configs and in case define its MIME type in `/etc/mime.types`.
 
 Monitoring
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