#5966 Add reliable checks for ambigious file extensions

[martgil]

This PR does fix .asc public key attachment not recognized.

Fix:

Current behavior:

Closes #5966

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Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)

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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

[martgil]

Hi @sosnovsky, this one is ready for a review. thanks!

[martgil]

Hi again @sosnovsky, I've added a small test for this change. Do you think it will be sufficient?

[sosnovsky]

all good now, thanks!