minor symfony#13843 Fill out custom type guesser paragraph (l-vo)

This PR was merged into the 4.4 branch.

Discussion
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Fill out custom type guesser paragraph

Fill out custom type guesser registration with the fullstack framework and add details about registration without the fullstack framework.

Commits
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d6757d9 Fill out custom type guesser paragraph

components/mime.rst

@@ -276,7 +276,7 @@ that extension to improve the guessing performance.
 Adding a MIME Type Guesser
 ..........................
 
-You can register your own MIME type guesser by creating a class that implements
+You can write your own MIME type guesser by creating a class that implements
 :class:`Symfony\\Component\\Mime\\MimeTypeGuesserInterface`::
 
     namespace App;
@@ -300,6 +300,55 @@ You can register your own MIME type guesser by creating a class that implements
         }
     }
 
+And registering it::
+
+    $mimeTypes = new MimeTypes();
+    $mimeTypes->registerGuesser(new SomeMimeTypeGuesser());
+
+When using the Symfony fullstack Framework, you just need to add the ``mime.mime_type_guesser`` tag:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/services.yaml
+        services:
+            App\SomeMimeTypeGuesser:
+                tags: [mime.mime_type_guesser]
+
+    .. code-block:: xml
+
+        <!-- config/services.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                https://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <services>
+                <service id="App\SomeMimeTypeGuesser">
+                     <tag name="mime.mime_type_guesser"/>
+                </service>
+            </services>
+        </container>
+
+    .. code-block:: php
+
+        // config/services.php
+        namespace Symfony\Component\DependencyInjection\Loader\Configurator;
+
+        use App\SomeMimeTypeGuesser;
+
+        return function(ContainerConfigurator $configurator) {
+            $services = $configurator->services();
+
+            $services->set(SomeMimeTypeGuesser::class)
+                ->tag('mime.mime_type_guesser');
+        };
+
+Note that this is already done for you if you use the :ref:`default services.yaml configuration <service-container-services-load-example>`
+thanks to :ref:`autoconfigure <services-autoconfigure>`.
+
 .. _`MIME`: https://en.wikipedia.org/wiki/MIME
 .. _`MIME types`: https://en.wikipedia.org/wiki/Media_type
 .. _`fileinfo extension`: https://www.php.net/fileinfo