[FIX] mail: handle emails with a single non-text part

SAP sends out emails that only contain a PDF part, without a text or
html body, e.g.:

    X-Mailer: SAP NetWeaver 751
    Content-Type: application/pdf; name="Afleveringsbewijs.pdf"
    Content-Transfer-Encoding: base64
    Content-Description: Afleveringsbewijs

Before this patch, this single non-text part was parsed into the body of
the final message, as if it was a text part.

Making the check for the main part more restrictive solves the issue, as
there is no reason to assume that a non-multipart content is
necessarily of text/* type. This PR also includes a regression test.

Closes #21102

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