Sending multi-part email sends HTML part as attachement

Question

I'm using micronaut and I want to send template e-mails.
But when I send e-mail the HTML part of the email is sent as attachement.

I use SMTP office 365 server for outgoing e-mails.
I have this piece of code and I tried different SMTP servers and also recievers.

    fun sendMail(email: EmailData): EmailException? {
        try {
            emailSender.send(
                Email.builder()
                    .from(email.from)
                    .to(email.to)
                    .subject(email.subject)
                    .body(
                        MultipartBody(
                            TemplateBody(BodyType.HTML, ModelAndView("email/purchase-created-html", email.variables)),
                            TemplateBody(BodyType.TEXT, ModelAndView("email/purchase-created-text", email.variables))
                        )
                    )
            )
            return null
        } catch (exception: EmailException) {
            return exception
        }

    }

Do you know why is the HTML part sent as html attachement? The content-type of e-mail is multipart