Add external links for tasks with plugin api

tests/units/ExternalLink/AttachmentLinkTest.php

@@ -0,0 +1,18 @@
+<?php
+
+require_once __DIR__.'/../Base.php';
+
+use Kanboard\ExternalLink\AttachmentLink;
+
+class AttachmentLinkTest extends Base
+{
+    public function testGetTitleFromUrl()
+    {
+        $url = 'https://kanboard.net/folder/document.pdf';
+
+        $link = new AttachmentLink($this->container);
+        $link->setUrl($url);
+        $this->assertEquals($url, $link->getUrl());
+        $this->assertEquals('document.pdf', $link->getTitle());
+    }
+}