improve named part/attachment detection

[ashiire]

This will add filename detection for parts which specify Content-Disposition without a filename parameter and Content-Type with a name parameter. I'm not sure whether omitting this possibility was intentional, e.g. to adhere to standard, but messages which name their attachments in this way occur in the wild.

[ashiire]

Fabricated example of the behavior in question.

$ ls mail_s_poezii.txt | mshow -t
mail_s_poezii.txt
  1: multipart/mixed size=825
    2: text/plain size=30
    3: text/plain size=53 name="kocka_leze_dirou.txt"
    4: text/plain size=98
$ ls mail_s_poezii.txt | mshow-patched -t
mail_s_poezii.txt
  1: multipart/mixed size=825
    2: text/plain size=30
    3: text/plain size=53 name="kocka_leze_dirou.txt"
    4: text/plain size=98 name="za_pravdu_je_ruzna_sazba.txt"

file contents:

From: Petr Klic <p.klic@example.com>
To: <umeni@example.com>
Subject: Poezie
Date: Wed, 14 Oct 2020 12:18:08 +0200
Message-ID: <setrise3750@example.com>
MIME-Version: 1.0
Content-Type: multipart/mixed;
        boundary="----=_NextPart_000_00DE_01D6A2E8.A7446C80"
Content-Language: cs

This is a multipart message in MIME format.

------=_NextPart_000_00DE_01D6A2E8.A7446C80
Content-Type: text/plain;
        charset="UTF-8"
Content-Transfer-Encoding: 8bit

Dobry den,

posilam poezii.

------=_NextPart_000_00DE_01D6A2E8.A7446C80
Content-Type: text/plain;
        charset="UTF-8";
        name="kocka_leze_dirou.txt"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment;
        filename="kocka_leze_dirou.txt"

kocka leze dirou
pes oknem
nebude-li prset
nezmoknem

------=_NextPart_000_00DE_01D6A2E8.A7446C80
Content-Type: text/plain;
        charset="UTF-8";
        name="za_pravdu_je_ruzna_sazba.txt"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment

za pravdu je ruzna sazba
nekdy pomnik
jindy vazba
kdo ji rika driv nez vcas
tomu pravda
zlame vaz

------=_NextPart_000_00DE_01D6A2E8.A7446C80--

[ashiire]

Please let me know if there are any problems with the patch so I can try to correct them.

To clarify, I believe the current behavior is a problem because in the affected emails I have come across, these mimeparts are clearly intended to be treated as attachments and appear to be treated as such by other MUAs, but not mblaze.

[leahneukirchen]

This looks good and probably is what we intended in the first place.

Please remove the spurious whitespace in one line, and if you can, add a test that checks all possible 4 situations.