Fix sending encrypted messages with Mailvelope

[pabzm]

Steps to reproduce

Send an encrypted message using Mailvelope with the Mail app. View the message in other MUAs than Mail+Mailvelope.

Expected behavior

See the inputted message text.

Actual behavior

See mime-headers and the encoded message text.

Mail app version

latest main

Mailserver or service

any

Operating system

all

PHP engine version

None

Web server

None

Database

None

Additional info

Mailvelope's API to encrypt messages returns a ciphertext that is supposed to be sent as a pgp/mime-message, i.e. with the message header line Content-Type: multipart/encrypted; protocol="application/pgp-encrypted".

Mail sends the ciphertext as text/plain, though: Content-Type: text/plain; charset=utf-8.

As a result, a recipient e.g. using Thunderbird gets to see mime-structured data as text after decryption:

Content-Type: multipart/mixed;
 boundary="----sinikael-?=_1-17211123607640.9888785964550162"
Date: Tue, 16 Jul 2024 06:46:00 +0000
Message-Id: <something>
MIME-Version: 1.0

------sinikael-?=_1-17211123607640.9888785964550162
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

=F0=9F=A4=AF
------sinikael-?=_1-17211123607640.9888785964550162--

Mailvelope itself appears to interpret the data instead of showing it, but other MUAs don't (and aren't supposed to).

[pabzm]

Note that fixing this problem will put more pressure on #3833, because the correctly formatted messages won't be decryptable using the Mail app with Mailvelope until that other issue is fixed.