[FIX] mail_thread: save attachment from mail in same encoding

Steps to reproduce:

  - Configure incoming mail server and set it to create X record
    on incoming mails (X can be any model with a chatter)
  - Create a CSV file and set the encoding to UTF-16
  - Send the CSV file through Gmail to the Odoo instance
  - Go to model X and open the created record
  - In the chatter, click/download the CSV file
  - Open the downloaded file with Geany (or any file editor that can
    show the file encoding)

Issue:

  The file encoding is not the same as the original file (utf-8 instead
  of utf-16).
  Working with Outlook.

Cause:

  The difference between Outlook and Gmail is that Gmail provides the
  charset of the file.

  The content of the mail is retrieved using `email` python lib.
  The lib will try to retrieve the charset of the file and fallback
  on `ASCII` if not available, then return the decode content.

```python
  def get_text_content(msg, errors='replace'):
    content = msg.get_payload(decode=True)
    charset = msg.get_param('charset', 'ASCII')
    return content.decode(charset, errors=errors)
```

  Example:
  content = b'd\x00a\x00,\x00,\x00,\......'
  Outlook:
  charset = 'ASCII'
  return => 'd\x00a\x00,\x00,\x00...'
  Gmail:
  charset = 'UTF-16LE'
  return => 'da,,,,,\n,,,,,\....'

  In the post process of the attachment, the content is encoded in
  'utf-8' (to then encoded in base64) before creating the attachment
  record.

  Content encoded to 'utf-8':
  Outlook: b'd\x00a\x00,\x00,\x00...'
  Gmail:  b'da,,,,,\n,,,,,\n....'

  Therefore, when writing the file on the disk, the encoding is based
  on the binary content.

Solution:

  When parsing the mail, add the encoding charset to the `info` variable.
  Then, when creating the attachment, use the charset in `info` (or
  fallback on 'utf'8' if no charset set) to encode the content.

opw-3089009

closes odoo/odoo#111298

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>