[ISSUE] Confusing mshow behaviour with multipart/form-data

[Earnestly <Earnestly@users.noreply.github.com>]

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New issue by Earnestly on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/212

Description:
Version: https://github.com/leahneukirchen/mblaze/commit/4ccf2f08c1aa8b15f31ac469edebe6c4710d74f1

For full disclosure I'm attempting to reuse `mshow` for POST data instead of email so perhaps this expected but I was hoping you may have some insight into this behaviour I'm witnessing.

Here is an example file I am working with, it included CRLFs (I'm not sure if github strips them):

```
Content-Type: multipart/form-data; boundary=------------------------55a586f81559face

--------------------------55a586f81559face
Content-Disposition: form-data; name="a"; filename="foo"
Content-Type: application/octet-stream

foo

--------------------------55a586f81559face
Content-Disposition: form-data; name="a"; filename="bar"
Content-Type: application/octet-stream

bar

--------------------------55a586f81559face--
```

When in this form it appears to work as I would expect:

```
% mshow -t - < example
/dev/stdin
  1: multipart/form-data size=346
    2: application/octet-stream size=4 name="foo"
    3: application/octet-stream size=4 name="bar"
```

However if any of the content contains a newline it appears to throw `mshow` off.

```
Content-Type: multipart/form-data; boundary=------------------------55a586f81559face

--------------------------55a586f81559face
Content-Disposition: form-data; name="a"; filename="foo"
Content-Type: application/octet-stream

foo

baz ^ newline

--------------------------55a586f81559face
Content-Disposition: form-data; name="a"; filename="bar"
Content-Type: application/octet-stream

bar

--------------------------55a586f81559face--
```

```
% mshow -t - < example
/dev/stdin
  1: multipart/form-data size=212 name="foo"
    2: application/octet-stream size=4 name="bar"

% mshow -x - < example
foo
bar

%head -n-0 foo bar
==> foo <==
baz ^ newline

--------------------------55a586f81559face
Content-Disposition: form-data; name="a"; filename="bar"
Content-Type: application/octet-stream

bar

--------------------------55a586f81559face--

==> bar <==
bar
```

If I include a newline in `bar` then `mshow` would produce an empty file.

Am I doing anything wrong or is `mshow` not an appropriate for POST multipart/form-data?