wp-includes/PHPMailer/PHPMailer.php:3401Set the message type.
protected function setMessageType() { $type = []; if ($this->alternativeExists()) { $type[] = 'alt'; } if ($this->inlineImageExists()) { $type[] = 'inline'; } if ($this->attachmentExists()) { $type[] = 'attach'; } $this->message_type = implode('_', $type); if ('' === $this->message_type) { //The 'plain' message_type refers to the message having a single body element, not that it is plain-text $this->message_type = 'plain'; } }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.