FileValidationConstraintValidator.php

<?php

namespace Drupal\file\Plugin\Validation\Constraint;

use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Checks that a file referenced in a file field is valid.
 */
class FileValidationConstraintValidator extends ConstraintValidator {
    
    /**
     * {@inheritdoc}
     */
    public function validate($value, Constraint $constraint) {
        // Get the file to execute validators.
        $target = $value->get('entity')
            ->getTarget();
        if (!$target) {
            return;
        }
        $file = $target->getValue();
        // Get the validators.
        $validators = $value->getUploadValidators();
        // Checks that a file meets the criteria specified by the validators.
        if ($errors = file_validate($file, $validators)) {
            foreach ($errors as $error) {
                $this->context
                    ->addViolation($error);
            }
        }
    }

}