bug#50154: [Rmail]: Cannot save attachments from decryped email

[Ariel Costas via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

When I receive an unencrypted email on my inbox and view it through
Rmail, I get the option to save the file.

     [1:text/plain Show]

     [2:image/jpeg Show Save:My_attachment.jpg (400kB)]

However, when I receive an encrypted message, and decrypt it with `M-x
rmail-epa-decrypt`, I get to see the plaintext of all the email, with
the multipart/mixed Content-Type, the email's body and then another part
with the attachment as base64, without an option to save it to my disk.

    Content-Type: multipart/mixed;boundary=---------------------BOUNDARY

    -----------------------BOUNDARY
    Content-Transfer-Encoding: quoted-printable
    Content-Type: text/plain;charset=utf-8

    The body of the message
    -----------------------BOUNDARY
    Content-Type: mime/type; filename="My_attachment.format"; name="My_attachment.format"
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename="My_attachment.jpg"; name="My_attachment.format"

Apparently, when decrypting the email, Rmail doesn't parse its contents
to highlight it or show the attachments as on the first scenario.

--
Ariel Costas https://costas.dev

Encrypt your email -- https://emailselfdefense.org/
  GnuPG public key -- https://costas.dev/pgp