Missing attachment when email is not multipart and only contains an attachment

## Describe the bug

When an email has no text or html content and is not multipart and **only** contains an attachment, postal doesn't show the attachment in the webUI.

## To Reproduce

1. Send the attached email
2. Click on incoming messages, choose the email
3. Click on Attachments
4. See "There are no attachments for this message."

## Expected behaviour

For the attachment to be displayed and downloadable in the webUI

## Environment details

- OS: n/a
- Browser n/a
- Version : 2.1.4
- Type : n/a

## Additional information/context

[postal_example.eml.gz](https://github.com/postalserver/postal/files/13725772/postal_example.eml.gz)