#5726 Improve encrypted message detection for multipart/encrypted email

[martgil]

This PR Improves detection for an multipart/encrypted emails with application/pgp-encrypted parts.

close #5726

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Tests (delete all except exactly one):

• Tests added or updated - I have asked the sender of the test email if we can incorporate the sent test email to our codebase.

Edit: I have been able to replace the sensitive part of the email while still preserving the main email structure of a multipart/encrypted email so asking for the actual email is not necessary needed.

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• [x] addresses the issue it closes (if any)
• [x] code is readable and understandable
• [x] is accompanied with tests, or tests are not needed
• [x] is free of vulnerabilities
• [x] is documented clearly and usefully, or doesn't need documentation

[martgil]

Hello @sosnovsky, this is now ready for a review.

It's also been helpful if we could share a build for him. But generally speaking, the bug is fixed and the "noname" multipart/encrypted where successfully recognized.

I'll be crafting a custom build and share it with them once I have your approval. Thank you!