Default headers for plain text mail content

• For plain text content of an email, these default headers are added:
  • CONTENT-TYPE: text/html; charset=utf-8
  • CONTENT-TRANSFER-ENCODING: 7bit
  • CONTENT-DISPOSITION: inline
• These headers can be overwritten by the user with the additionalHeaders dictionary
  • This is usually not recommended as any headers in additionalHeaders are applied to the entire email, so multi content-type emails will be rendered incorrectly. This will be noted in the docs.
• Additional changes to the other content types (mixed, alternative, related) will also be made so that the default CONTENT-TYPE header can be overwritten as well (but again, not recommended)