OpenPGP encrypted email from Thunderbird is decrypted into a multipart format that's not handled transparently

When a Thunderbird user sends an OpenPGP encrypted email to a VM user, and the VM user decrypts it with epa-mail-decrypt, the decrypted email appears in a multipart format with the message body buried in one section. Instead, VM should present just the email's body to the VM user.

Here is an example decrypted email after the following actions happen:

  1. The VM and Thunderbird user exchange public keys and import them into their respective programs.
  2. The Thunderbird user sends an encrypted message with body "Hello world!" to the VM user.
  3. The VM user receives the message.
  4. The VM user runs vm-decode-mime-message (keypress "D") to display the encrypted message in the vm Presentation buffer, instead of displaying buttons. (The epa-mail-decrypt function won't recognize the buttons as an encrypted message. This may be an additional bug.)
  5. The VM user runs epa-mail-decrypt.
  6. The message body below appears. Notice that the body "Hello world" is buried in a subsection.

VM is version 8.2.0b.

Thunderbird is version 128.5.2esr on the Mac.

This is an OpenPGP/MIME encrypted message (RFC 4880 and 3156)
--------------zL1zHOJY30mO3iRo9i7N8qdL
Content-Type: application/pgp-encrypted
Content-Description: PGP/MIME version identification

Version: 1

--------------zL1zHOJY30mO3iRo9i7N8qdL
Content-Type: application/octet-stream; name="encrypted.asc"
Content-Description: OpenPGP encrypted message
Content-Disposition: inline; filename="encrypted.asc"

Content-Type: multipart/mixed; boundary="------------Q42NxzOscUOgwz0h0tnzKNee";
 protected-headers="v1"
Subject: test encryption
From: Thunderbird person <email-here@whatever.com>
To: VM User <vmuser@whatever.com>
Message-ID: <b7a0289d-ed4d-41c8-89c4-44b075d27ea6@whatever.com>
References: <26474.45976.753806.379233@whatever.com>
In-Reply-To: <26474.45976.753806.379233@whatever.com>

--------------Q42NxzOscUOgwz0h0tnzKNee
Content-Type: multipart/mixed; boundary="------------egutjd3rZ0CxEKL1UeaB80si"

--------------egutjd3rZ0CxEKL1UeaB80si
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Hello world!

--------------egutjd3rZ0CxEKL1UeaB80si
Content-Type: application/pgp-keys; name="OpenPGP_0x8BF061FEE2712A7F.asc"
Content-Disposition: attachment; filename="OpenPGP_0x8BF061FEE2712A7F.asc"
Content-Description: OpenPGP public key
Content-Transfer-Encoding: 7bit

-----BEGIN PGP PUBLIC KEY BLOCK-----

xsDNBGdqrVUBDAC5ksTT3UTxsPiGCsa2o67Yb+ymCmEzN85bS4CvdSybpev+jWQC
...key details omitted...
-----END PGP PUBLIC KEY BLOCK-----

--------------egutjd3rZ0CxEKL1UeaB80si--
--------------Q42NxzOscUOgwz0h0tnzKNee--
--------------zL1zHOJY30mO3iRo9i7N8qdL--
Edited by Daniel Barrett