Add simple implementation (command handler) for avatar upload.

src/Core/Handlers/Commands/UploadAvatarCommandHandler.php

@@ -0,0 +1,59 @@
+<?php namespace Flarum\Core\Handlers\Commands;
+
+use Flarum\Core\Commands\UploadAvatarCommand;
+use Flarum\Core\Events\AvatarWillBeUploaded;
+use Flarum\Core\Repositories\UserRepositoryInterface;
+use Flarum\Core\Support\DispatchesEvents;
+use Illuminate\Support\Str;
+use League\Flysystem\Adapter\Local;
+use League\Flysystem\FilesystemInterface;
+use League\Flysystem\MountManager;
+
+class UploadAvatarCommandHandler
+{
+    use DispatchesEvents;
+
+    /**
+     * @var UserRepositoryInterface
+     */
+    protected $users;
+
+    /**
+     * @var FilesystemInterface
+     */
+    protected $uploadDir;
+
+    public function __construct(UserRepositoryInterface $users, FilesystemInterface $uploadDir)
+    {
+        $this->users = $users;
+        $this->uploadDir = $uploadDir;
+    }
+
+    public function handle(UploadAvatarCommand $command)
+    {
+        $user = $this->users->findOrFail($command->userId);
+
+        // Make sure the current user is allowed to edit the user profile.
+        // This will let admins and the user themselves pass through, and
+        // throw an exception otherwise.
+        $user->assertCan($command->actor, 'edit');
+
+        $filename = $command->file->getFilename();
+        $uploadName = Str::lower(Str::quickRandom()) . '.jpg';
+
+        $mount = new MountManager([
+            'source' => new Local($command->file->getPath()),
+            'target' => $this->uploadDir,
+        ]);
+
+        $user->changeAvatarUrl($uploadName);
+
+        event(new AvatarWillBeUploaded($user, $command));
+
+        $mount->move("source://$filename", "target://$uploadName");
+        $user->save();
+        $this->dispatchEventsFor($user);
+
+        return $user;
+    }
+}