net.smtp: attachments seem like not fully conform for all mail providers

[7underlines]

Describe the bug

It is not possible to send an email with both a body and an attachment to someone using outlook.office.com:

• Sending an email with a body only works fine: the receiver sees the email body.
• Sending an email with both a body and an attachment results in the receiver seeing only the attachment.

With gmail everything works fine.

From one GMX user I heard that attachments are missing, but I haven't checked it myself.

Reproduction Steps

Send email with attachment as described here #20640

Expected Behavior

Receiver gets email body and attachment.

Current Behavior

Receiver gets only the email attachment.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.9 40b574b

Environment details (OS name and version, etc.)

V full version	V 0.4.9 fe5609d.40b574b
OS	linux, Pop!_OS 22.04 LTS
Processor	8 cpus, 64bit, little endian, Intel(R) Core(TM) i5-8305G CPU @ 2.80GHz
Memory	2.68GB/15.34GB
V executable	/home/loguser002/a/v/v
V last modified time	2025-01-16 12:57:18
V home dir	OK, value: /home/loguser002/a/v
VMODULES	OK, value: /home/loguser002/.vmodules
VTMP	OK, value: /tmp/v_1000
Current working dir	OK, value: /home/loguser002/a/rosenapotheke-importv2
Git version	git version 2.34.1
V git status	weekly.2025.1-64-g40b574b4
.git/config present	true
cc version	cc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
gcc version	gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
clang version	N/A
tcc version	tcc version 0.9.28rc 2024-07-31 HEAD@1cee0908 (x86_64 Linux)
tcc git status	thirdparty-linux-amd64 0134e9b9
emcc version	N/A
glibc version	ldd (Ubuntu GLIBC 2.35-0ubuntu3.8) 2.35

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