SUMMARY
When view/header/standard is configured some attachments are not shown in the email window, so that there is no indication of an attachment there. Switching to view/header/decorative shows the attachments in the same email window's header section.

Example code for not shown attachment with standard header ([UUID] is a placeholder):
--Apple-Mail=_[UUID]
Content-Disposition: inline;
        filename="filename.pdf"
Content-Type: application/pdf;
        x-unix-mode=0644;
        name="filename.pdf"
Content-Transfer-Encoding: base64

[code]

Example code for shown attachment with standard header:
--multipart_alternative.123456789--

--multipart_mixed.987654321
Content-Type: application/pdf
Content-Length: 42000
Content-Disposition: attachment; filename="filename.pdf"
Content-Transfer-Encoding: base64

[code]

OBSERVED RESULT
No indication of an attachment in the email window.

EXPECTED RESULT
An attachment is shown in standard view in all emails

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.19.5
KDE Frameworks Version: 5.74.0
Qt Version: 5.15.2