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The whitepaper (https://www.whatsapp.com/security/WhatsApp-Security-Whitepap...) claims that the attachments of any type are encrypted

It definitely should include file transfers, according to their blog post: https://blog.whatsapp.com/10000618/end-to-end-encryption.

Are you sure encryption was active, both devices on the latest version? Is there a chance that the conversion was done on either of the devices?

Otherwise, yeah, something isn't right.

Conversion is done by the devices I think.