#5458 Improve parsing of pgp/mime message with attachment

[sosnovsky]

This PR fixes parsing of large PGP/MIME encrypted messages with attachment, which previously were not detected as encrypted ones.

close #5458

before (original message isn't detected as encrypted and rendered as plain text):

after (encrypted message is parsed and correctly renders decryption error):

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Tests (delete all except exactly one):

• Difficult to test (explain why) - it happens just for some messages, wasn't able to recreate similar pgp/mime message

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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):

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

[sosnovsky]

@martgil this one ready for review, thanks!

[martgil]

@sosnovsky does the above email from the screenshot available to our compatibility email account? Perhaps I can review it and potentially generate a test email for this one?

[sosnovsky]

@sosnovsky does the above email from the screenshot available to our compatibility email account? Perhaps I can review it and potentially generate a test email for this one?

Yes, it's in flowcrypt.compatibility account - thread Re: FES message for #5458 from October 16

[martgil]

Got it - thank you, Roma.

[sosnovsky]

By the way, original message was created using FES 2023-11, the same message from FES 2023-12 doesn't have such issue. Also it happens only in Gmail inbox, while in FlowCrypt Encrypted Inbox message is rendered correctly even without current fix.

[martgil]

Okay, I did try to create a test email, but I think it is difficult to do so. The issue is, it must be long enough to trigger Gmail's email body message clipping. This means that the non-encrypted data would include large characters, which the browser extension cannot handle, making it difficult to create a test email. That would be a different story since the above screenshot results in a decryption error (probably because it was an encrypted email for another key).

I have tested it and it should work on other instances wherein the decrypted content is not very long but the ASCII armored is long enough to trigger Gmail's email body message clipping feature.