example_3_guesser.php

// Kernel.php
<?php
namespace App;

use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Reference;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;
use Symfony\Component\Mime\FileBinaryMimeTypeGuesser;

class Kernel extends BaseKernel implements CompilerPassInterface
{
    use MicroKernelTrait;

    public function process(ContainerBuilder $container)
    {
        // We should guess the mimetype from the FileBinaryMimeTypeGuesser first.
        // We register it as a service
        $container->register(FileBinaryMimeTypeGuesser::class, FileBinaryMimeTypeGuesser::class);
        $container
            // We get the service MimeTypes that is aliased by 'mime_types'
            ->getDefinition('mime_types')
            // we call the method 'registerGuesser' on the MimeTypes with parameterFileBinaryMimeTypeGuesser 
            ->addMethodCall('registerGuesser', [new Reference(FileBinaryMimeTypeGuesser::class)])
        ;
    }
}

// FileCreator.php
use Symfony\Component\Mime\{MimeTypes, FileBinaryMimeTypeGuesser};

class FileCreator
{
    private MimeTypesInterface $mimeTypes;
    
    public function __construct(MimeTypesInterface $mimeTypes)
    {
        $this->mimeTypes = $mimeTypes;
    }

    public function create(string $filePath): void
    {
        $mimeType = $this->mimeTypes->guessMimeType($filePath);

        if (false === $this->supports($mimeType)) {
            // throw error
        }
        // stuff
    }

    private function supports(?string $mimeType): bool
    {
       return 'application/pdf' === $mimeType;
    }
}