User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

Got an emails with an attachment.

Actual results:

In "plain text" mode, most of the time the paper-clip is shown.
But in some cases not.
The attachment icon ("paper-clip") is not shown when in "plain text" mode ("view->message_body_as->plain_text"). Switching to "simple html" or "html" all attachments icon are shown.

Expected results:

The attachment icon should be shown, regardless of the view mode.

Some more details:
using 66.0b2 (64-bit).

On other TB versions the attachment is shown.

Snippet from source:
$grep -B1 -A5 $sourceOfTheMail
--Apple-Mail=_$ID
Content-Disposition: attachment;
filename=smime.p7s
Content-Type: application/pkcs7-signature;
name=smime.p7s
Content-Transfer-Encoding: base64

Can you provide the message as an .eml file. Originating from Apple, this is likely a duplicate.

Here is the eml (sorry for the delay).
I stripped some data sensitive things and regexed some IDs, hope it is helpful nonetheless:

X-ListServer: CommuniGate Pro LIST 6.0.11
Precedence: list
MIME-Version: 1.0
Content-Type: multipart/signed;
boundary="Apple-Mail=_2C0hsupoa-1A10-4BB8-AE85-E2B3EE8FFD20";
protocol="application/pkcs7-signature";
micalg=sha-256
Date: Thu, 21 Feb 2019 20:27:59 +0100
Subject: ?
X-Spam-Flag: NO
X-UI-Filterresults: notjunk:1;V03:K0:KMR06qRnpxM=:
snip-snap

--Apple-Mail=_2C0hsupoa-1A10-4BB8-AE85-E2B3EE8FFD20
Content-Type: multipart/alternative;
boundary="Apple-Mail=_1EEB1F69-995D-4D3C-8B93-7892D789F8E9"

--Apple-Mail=_1EEB1F69-995D-4D3C-8B93-7892D789F8E9
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain;
charset=utf-8

here the text

--Apple-Mail=_1EEB1F69-995D-4D3C-8B93-7892D789F8E9
Content-Type: multipart/mixed;
boundary="Apple-Mail=_E90HGTU7-9B29-40D8-8313-19E444832E8E"

--Apple-Mail=_E90HGTU7-9B29-40D8-8313-19E444832E8E
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html;
charset=utf-8

here the html

--Apple-Mail=_E90HGTU7-9B29-40D8-8313-19E444832E8E
Content-Disposition: inline;
filename=Scan.pdf
Content-Type: application/pdf;
x-unix-mode=0644;
name="Scan.pdf"
Content-Transfer-Encoding: base64

here the pdf bytes

--Apple-Mail=_E90HGTU7-9B29-40D8-8313-19E444832E8E
Content-Transfer-Encoding: 7bit
Content-Type: text/html;
charset=us-ascii

some more html

--Apple-Mail=_E90HGTU7-9B29-40D8-8313-19E444832E8E--

--Apple-Mail=_1EEB1F69-995D-4D3C-8B93-7892D789F8E9--

--Apple-Mail=_2C0hsupoa-1A10-4BB8-AE85-E2B3EE8FFD20
Content-Disposition: attachment;
filename=smime.p7s
Content-Type: application/pkcs7-signature;
name=smime.p7s
Content-Transfer-Encoding: base64

bytes

--Apple-Mail=_2C0hsupoa-1A10-4BB8-AE85-E2B3EE8FFD20--

Exactly what I said: The attachment sits in one alternative part and is thus not detected when the other alternative part, the plain text, is viewed. Bug 1362539 ... attachment sits in alternative part.