Add simple implementation (command handler) for avatar upload.

src/Core/Commands/UploadAvatarCommand.php

@@ -0,0 +1,30 @@
+<?php namespace Flarum\Core\Commands;
+
+use RuntimeException;
+
+class UploadAvatarCommand
+{
+    public $userId;
+
+    /**
+     * @var \Symfony\Component\HttpFoundation\File\UploadedFile
+     */
+    public $file;
+
+    public $actor;
+
+    public function __construct($userId, $file, $actor)
+    {
+        if (empty($userId) || !intval($userId)) {
+            throw new RuntimeException('No valid user ID specified.');
+        }
+
+        if (is_null($file)) {
+            throw new RuntimeException('No file to upload');
+        }
+
+        $this->userId = $userId;
+        $this->file = $file;
+        $this->actor = $actor;
+    }
+}