check if file isValid()

the `isValid()` method checks whether the file was uploaded successfully, and also calls `is_uploaded_file()`, which validates the file has been uploaded, and helps prevent mailcious users from reading other files on the system.

there is no need to check for the file existence because the constructor on `Symfony\Component\HttpFoundation\File\File` checks if the passed path is a file using `is_file()`.

also updated test

UploadedFile.php

@@ -95,7 +95,7 @@ public function storeAs($path, $name, $options = [])
      */
     public function getContents()
     {
-        if (file_exists($this->getPathname())) {
+        if ($this->isValid()) {
             return file_get_contents($this->getPathname());
         }