Attachments missing

Forwarded mail: I get attachments Bitmask mail: attachments are missing in Thunderbird, but show up in Pixelated

2017-08-08T23:55:14+0200 [twisted.python.log#info] "127.0.0.1" - - [08/Aug/2017:21:55:13 +0000] "GET /mails?q=tag%3A%22inbox%22&p=1&w=25 HTTP/1.1" 200 104912 "http://localhost:9090/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/538.1 (KHTML, like Gecko) Qt/5.5.1 Safari/538.1"
2017-08-08T23:55:14+0200 [twisted.python.log#info] "127.0.0.1" - - [08/Aug/2017:21:55:13 +0000] "GET /tags HTTP/1.1" 200 415 "http://localhost:9090/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/538.1 (KHTML, like Gecko) Qt/5.5.1 Safari/538.1"
2017-08-08T23:55:19+0200 [twisted.python.log#info] "127.0.0.1" - - [08/Aug/2017:21:55:18 +0000] "GET /mails?q=tag%3A%22inbox%22&p=1&w=25 HTTP/1.1" 200 104912 "http://localhost:9090/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/538.1 (KHTML, like Gecko) Qt/5.5.1 Safari/538.1"

When I view the source in TB, the attachments do show up there. Multipart is missing, don't know which part is responsible for that? Original mail starts like this:

[…]
X-Virus-Scanned: clamav-milter 0.99.2 at pangolin
X-Virus-Status: Clean
X-Spam-Status: No, score=-0.1 required=3.5 tests=DKIM_SIGNED,DKIM_VALID,
	DKIM_VALID_AU autolearn=disabled version=3.4.0
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	evasione.investici.org
--St7VIuEGZ6dlpu13
Content-Type: multipart/mixed; boundary="cNdxnHkX5QqsyA0e"
Content-Disposition: inline


--cNdxnHkX5QqsyA0e
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable
I've attached the spec files and design docs.
[…]

Mail inside Bitmask starts like this:

[…]
Content-Type: multipart/signed; micalg=pgp-sha512;
 protocol="application/pgp-signature"; boundary="St7VIuEGZ6dlpu13"
X-Virus-Status: Clean
X-Leap-Signature: valid; pubkey="F47351BD87AB7FCF6F8880C9836501BE9F27A723"
Mime-Version: 1.0


--St7VIuEGZ6dlpu13
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline


I've attached the spec files and design docs.
[…]