Support cid URLs as used in mailstream plugin

src/Content/Text/BBCode.php

@@ -28,6 +28,7 @@ use Friendica\Content\ContactSelector;
 use Friendica\Content\Item;
 use Friendica\Content\OEmbed;
 use Friendica\Content\Smilies;
+use Friendica\Content\Text\HTMLPurifier_URIScheme_cid;
 use Friendica\Core\Hook;
 use Friendica\Core\Logger;
 use Friendica\Core\Protocol;
@@ -1874,6 +1875,8 @@ class BBCode
 			$text
 		);
 
+		\HTMLPurifier_URISchemeRegistry::instance()->register('cid', new HTMLPurifier_URIScheme_cid());
+
 		$config = \HTMLPurifier_HTML5Config::createDefault();
 		$config->set('HTML.Doctype', 'HTML5');
 		$config->set('HTML.SafeIframe', true);

src/Content/Text/HTMLPurifier_URIScheme_cid.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace Friendica\Content\Text;
+
+use \HTMLPurifier_URIScheme;
+
+/**
+ * Validates content-id ("cid") as used in multi-part MIME messages, as defined by RFC 2392
+ */
+class HTMLPurifier_URIScheme_cid extends HTMLPurifier_URIScheme
+{
+	/**
+	 * @type bool
+	 */
+	public $browsable = true;
+
+	/**
+	 * @type bool
+	 */
+	public $may_omit_host = true;
+
+	/**
+	 * @param HTMLPurifier_URI $uri
+	 * @param HTMLPurifier_Config $config
+	 * @param HTMLPurifier_Context $context
+	 * @return bool
+	 */
+	public function doValidate(&$uri, $config, $context)
+	{
+		$uri->userinfo = null;
+		$uri->host = null;
+		$uri->port = null;
+		$uri->query = null;
+		// typecode check needed on path
+		return true;
+	}
+}