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That file format says that your email was digitally signed. The signature shows as a file attached to your email so what Amazon received likely included the epub and the p7s files as attachments. I would have expected that Amazon's servers should be bright enough to disregard the p7s file as a file attachment but live and be disappointed. You could try disabling signing email going to to your Kindle depending on how your email client configures that setting.
Have you tried using calibre to send the ebook using an email server? I've been using GMX for the purpose, you just need to remember to enable POP3 & IMAP under Settings => POP3 & IMAP from the GMX webpage.
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