[6.x] Fix setting mail header

[driesvints]

Replaces #32271

[ghost]

Hi,
We've literally just found an issue with this PR, where if you add an attachment (ours was a calendar ics file) then the html email body is sent as raw html. When we downgraded from 6.18.4 to 6.18.3 this issue was resolved. Thought you would like to know. Here is a screengrab of what we were getting in 6.18.4

Hope that helps;
The mail was run using the following command (filenames removed for security);
public function build() { return $this->view('xxxxxxxx')->attachData($this->ics, 'xxxxx.ics', ['mime' => "text/calendar"]); }

[driesvints]

@bengallienne We're reverting this change. Thanks for notifying.