Use Mime component to determine mime type for file validator

src/Symfony/Component/Validator/Constraints/FileValidator.php

@@ -13,8 +13,10 @@
 
 use Symfony\Component\HttpFoundation\File\File as FileObject;
 use Symfony\Component\HttpFoundation\File\UploadedFile;
+use Symfony\Component\Mime\MimeTypes;
 use Symfony\Component\Validator\Constraint;
 use Symfony\Component\Validator\ConstraintValidator;
+use Symfony\Component\Validator\Exception\LogicException;
 use Symfony\Component\Validator\Exception\UnexpectedTypeException;
 use Symfony\Component\Validator\Exception\UnexpectedValueException;
 
@@ -170,12 +172,12 @@ public function validate($value, Constraint $constraint)
         }
 
         if ($constraint->mimeTypes) {
-            if (!$value instanceof FileObject) {
-                $value = new FileObject($value);
+            if (!class_exists(MimeTypes::class)) {
+                throw new LogicException('You cannot validate the mime-type of files as the Mime component is not installed. Try running "composer require symfony/mime".');
             }
 
             $mimeTypes = (array) $constraint->mimeTypes;
-            $mime = $value->getMimeType();
+            $mime = $value instanceof FileObject ? $value->getMimeType() : MimeTypes::getDefault()->guessMimeType($path);
 
             foreach ($mimeTypes as $mimeType) {
                 if ($mimeType === $mime) {