Don't add .pgp to file list when attaching files on compose screen

[ekievsky]

This PR adds .pgp extension to attachments right before sending them

close #538

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Tests :

• 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

[tomholub]

Code looks good