recreating pull request nextcloud#12946

apps/dav/lib/CalDAV/Schedule/IMipPlugin.php

@@ -279,12 +279,18 @@ public function schedule(Message $iTipMessage) {
 
 		$message->useTemplate($template);
 
-		$attachment = $this->mailer->createAttachment(
-			$iTipMessage->message->serialize(),
-			'event.ics',// TODO(leon): Make file name unique, e.g. add event id
-			'text/calendar; method=' . $iTipMessage->method
+		/*
+		** We choose to work like Thunderbird Lightning.
+		** using a plain text text/calendar ics.
+		** This plain text text/calendar part is needed for
+		** Microsoft Outlook versions <= 2010 to work.
+		*/
+		$itip_msg = $iTipMessage->message->serialize();
+		$message->attachInline(
+				$itip_msg,
+				'text/calendar; method=' . $iTipMessage->method,
+				'UTF-8'
 		);
-		$message->attach($attachment);
 
 		try {
 			$failed = $this->mailer->send($message);

lib/private/Mail/Message.php

@@ -63,6 +63,26 @@ public function attach(IAttachment $attachment): IMessage {
 		return $this;
 	}
 
+	/**
+	 * @param $body:  body of the mime part
+	 * @param $content-type = null: Mime Content-Type (e.g. text/plain or text/calendar)
+	 * @param $charset = null: Character Set (e.g. UTF-8)
+	 * @return $this
+	 * @since 24.0.6
+	 */
+	public function attachInline($data, $contentType = null, $charset = null): IMessage {
+		# To be sure this works with iCalendar messages, we encode with 8bit instead of
+		# quoted-printable encoding. We save the current encoder, replace the current
+		# encoder with an 8bit encoder and after we've finished, we reset the encoder
+		# to the previous one.
+			$encoder = $this->swiftMessage->getEncoder();
+			$eightbitEncoder = new \Swift_Mime_ContentEncoder_PlainContentEncoder('8bit');
+			$this->swiftMessage->setEncoder($eightbitEncoder);
+			$this->swiftMessage->addPart($data, $contentType, $charset);
+			$this->swiftMessage->setEncoder($encoder);
+			return $this;
+	 }
+
 	/**
 	 * SwiftMailer does currently not work with IDN domains, this function therefore converts the domains
 	 * FIXME: Remove this once SwiftMailer supports IDN

lib/public/Mail/IMessage.php

@@ -40,6 +40,15 @@ interface IMessage {
 	 */
 	public function attach(IAttachment $attachment): IMessage;
 
+	/**
+	 * @param $body:  body of the mime part
+	 * @param $content-type = null: Mime Content-Type (e.g. text/plain or text/calendar)
+	 * @param $charset = null: Character Set (e.g. UTF-8)
+	 * @return IMessage
+	 * @since 24.0.6
+	 */
+	public function attachInline($body, $content_type = null, $charset = null): IMessage;
+
 	/**
 	 * Set the from address of this message.
 	 *