mimeparser: do not squash NDN text parts into attachments

Text part usually contains an error message that we want to display in
the UI.

test-data/message/ndn_with_attachment.eml

@@ -0,0 +1,123 @@
+Return-Path: <>
+Delivered-To: <alice@example.org>
+Date: Sat, 25 Jun 2022 21:35:33 -0400 (CDT)
+From: MAILER-DAEMON@example.org (Mail Delivery System)
+Subject: Undelivered Mail Returned to Sender
+To: alice@example.org
+Auto-Submitted: auto-replied
+MIME-Version: 1.0
+Content-Type: multipart/report; report-type=delivery-status;
+	boundary="C06EAE01B0.1656207333/relay01.example.org"
+Content-Transfer-Encoding: 7bit
+Message-Id: <20220626013533.5F0E5E0180@relay01.example.org>
+
+This is a MIME-encapsulated message.
+
+--C06EAE01B0.1656207333/relay01.example.org
+Content-Description: Notification
+Content-Type: text/plain; charset=us-ascii
+
+This is the mail system at host relay01.example.org.
+
+I'm sorry to have to inform you that your message could not
+be delivered to one or more recipients. It's attached below.
+
+For further assistance, please send mail to postmaster.
+
+If you do so, please include this problem report. You can
+delete your own text from the attached returned message.
+
+                   The mail system
+
+<bob@example.net>: host mx2.example.net[80.241.60.215] said: 552 5.2.2
+    <bob@example.net>: Recipient address rejected: Mailbox quota exceeded (in
+    reply to RCPT TO command)
+
+<bob2@example.net>: host mx1.example.net[80.241.60.212] said: 552 5.2.2
+    <bob2@example.net>: Recipient address rejected: Mailbox quota
+    exceeded (in reply to RCPT TO command)
+
+--C06EAE01B0.1656207333/relay01.example.org
+Content-Description: Delivery report
+Content-Type: message/delivery-status
+
+Reporting-MTA: dns; relay01.example.org
+X-Postfix-Queue-ID: C06EAE01B0
+X-Postfix-Sender: rfc822; alice@example.org
+Arrival-Date: Sat, 25 Jun 2022 21:35:23 -0400 (CDT)
+
+Final-Recipient: rfc822; bob@example.org
+Original-Recipient: rfc822;bob@example.org
+Action: failed
+Status: 5.2.2
+Remote-MTA: dns; mx2.example.net
+Diagnostic-Code: smtp; 552 5.2.2 <bob@example.org>: Recipient address
+    rejected: Mailbox quota exceeded
+
+Final-Recipient: rfc822; bob2@example.net
+Original-Recipient: rfc822;bob2@example.net
+Action: failed
+Status: 5.2.2
+Remote-MTA: dns; mx1.example.net
+Diagnostic-Code: smtp; 552 5.2.2 <bob2@example.net>: Recipient address
+    rejected: Mailbox quota exceeded
+
+--C06EAE01B0.1656207333/relay01.example.org
+Content-Description: Undelivered Message
+Content-Type: message/rfc822
+Content-Transfer-Encoding: 7bit
+
+Return-Path: <alice@example.org>
+Received: from smtp-gw01.enet.cu (unknown [172.29.8.31])
+	by relay01.example.org (Postfix) with ESMTP id C06EAE01B0;
+	Sat, 25 Jun 2022 21:35:23 -0400 (CDT)
+DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.org; s=nauta;
+	t=1656207323; bh=c0t8muX/2spIAQ/MRxSXsZ/ljSWP8uTxPcM5fqYC6qg=;
+	h=Subject:From:To:Date:In-Reply-To:References;
+	b=fggUmxnR3RjNfxO4S460fvxfenu3eUPvsGjldIGQeApxT508qGctJZ0tzYiFelOAu
+	 4zL8+VhlX0KdnrvRXo2/00O9U2kM8gpRLE9tIR0Rn6FyWM4nU5rcvAJp3oiUWxYzPt
+	 NMlzFeUsBvwFCJCEmZllSHaAyt/HTvWe68sGQsu8=
+Received: from amavis4.example.org (unknown [172.29.8.235])
+	by smtp-gw01.enet.cu (Postfix) with ESMTP id ECFE9180004D;
+	Sat, 25 Jun 2022 21:35:18 -0400 (CDT)
+Received: from smtp.example.org ([172.29.8.248])
+	by amavis4.example.org  with ESMTP id 25Q1ZNDW016368-25Q1ZNDX016368;
+	Sat, 25 Jun 2022 21:35:23 -0400
+Received: from [127.0.0.1] (unknown [10.59.196.114])
+	by smtp.example.org (Postfix) with ESMTPSA id E07E79F110;
+	Sat, 25 Jun 2022 21:35:21 -0400 (CDT)
+Subject: ...
+From: Alice <alice@example.org>
+To: <bob@example.net>, <bob2@example.net>
+Date: Sun, 26 Jun 2022 01:35:20 +0000
+Message-ID: <Mr.I6Da6dXcTel.TroC5J3uSDH@example.org>
+In-Reply-To: <Mr.I6Da6dXcTel.-qoK1IWpqby@example.org>
+References: <Mr.I6Da6dXcTel.TroC5J3uSDH@example.org>
+Chat-Version: 1.0
+MIME-Version: 1.0
+Content-Type: multipart/encrypted; protocol="application/pgp-encrypted";
+	boundary="OOGjcgxCkv3w8d1thY6jg0b8O0NFJE"
+
+
+--OOGjcgxCkv3w8d1thY6jg0b8O0NFJE
+Content-Type: application/pgp-encrypted
+Content-Description: PGP/MIME version identification
+
+Version: 1
+
+
+--OOGjcgxCkv3w8d1thY6jg0b8O0NFJE
+Content-Type: application/octet-stream; name="encrypted.asc"
+Content-Description: OpenPGP encrypted message
+Content-Disposition: inline; filename="encrypted.asc";
+
+-----BEGIN PGP MESSAGE-----
+
+REMOVED
+-----END PGP MESSAGE-----
+
+
+--OOGjcgxCkv3w8d1thY6jg0b8O0NFJE--
+
+
+--C06EAE01B0.1656207333/relay01.example.org--