Extract another middleware from API routing

src/Api/Middleware/ReadJsonParameters.php

@@ -0,0 +1,24 @@
+<?php namespace Flarum\Api\Middleware;
+
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Zend\Stratigility\MiddlewareInterface;
+
+class ReadJsonParameters implements MiddlewareInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function __invoke(Request $request, Response $response, callable $out = null)
+    {
+        if (str_contains($request->getHeaderLine('content-type'), 'application/vnd.api+json')) {
+            $input = json_decode($request->getBody(), true);
+
+            foreach ($input as $name => $value) {
+                $request = $request->withAttribute($name, $value);
+            }
+        }
+
+        return $out ? $out($request, $response) : $response;
+    }
+}