Add .eml example with odd behavior

[sftse]

This example exhibits strange behavior: mail-parser thinks it has a single attachment, although the only part of the email is plain/text and there is no multipart boundary. Thunderbird does a best-effort attempt at displaying the part as if it were text.

[sftse]

I've added a suggestion how to carve out this specific case.

[mdecimus]

Tests are failing for this PR.

[sftse]

Unclear how I missed the failing tests first time around. I've had to adapt the malformed/012.eml test case, but after manual review this seems indeed to be what I was going for. The handling of that case also appears more in line with Thunderbird, Thunderbird sees three .eml attachments, each without attachment. mail-parser only recognizes two attachments on the root email, but given the test case is marked as malformed, this difference might better be resolved in a different PR.