Support PNG avatars with transparent backgrounds and fix EXIF rotation (#1168)

As `orientate` requires the EXIF extension, we can only call it if the extension is installed.

Fixes #1161 and #1163.

src/Core/Command/RegisterUserHandler.php

@@ -173,7 +173,7 @@ class RegisterUserHandler
             'target' => $this->uploadDir,
         ]);
 
-        $uploadName = Str::lower(Str::quickRandom()).'.jpg';
+        $uploadName = Str::lower(Str::quickRandom()).'.png';
 
         $user->changeAvatarPath($uploadName);