#5629 Fix decryption error for ambiguous noname file with multipart/mixed email attachment.

[martgil]

This PR fixes decryption errors that occur on plain emails with ambiguous noname files with multipart/mixed email attachments.

close #5629

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Tests (delete all except exactly one):

• Difficult to test - The user just received the email, so I think it's something we can't ask to be part of the test. Additionally, I'm not sure how to generate an email with such an attachment. I checked if I can trim down the sensitive part from the noname file, but I think it's not possible and can't be done easily. This can be manually tested through user confirmation, though it works fine for me with the changes in this PR.

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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]

I've shared a build of this PR with the affected user and they have confirmed that the issue is no longer persists.

[martgil]

Hi @sosnovsky, just a small fix. ready for review. thanks!