Zip Exports: Added attachment/image link resolving & JSON null handling

app/References/ModelResolvers/AttachmentModelResolver.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace BookStack\References\ModelResolvers;
+
+use BookStack\Uploads\Attachment;
+
+class AttachmentModelResolver implements CrossLinkModelResolver
+{
+    public function resolve(string $link): ?Attachment
+    {
+        $pattern = '/^' . preg_quote(url('/attachments'), '/') . '\/(\d+)/';
+        $matches = [];
+        $match = preg_match($pattern, $link, $matches);
+        if (!$match) {
+            return null;
+        }
+
+        $id = intval($matches[1]);
+
+        return Attachment::query()->find($id);
+    }
+}