I've sent an email with an MS Word document as an attachment. When I look at the email on the activity log and open the attachment with Word then I get a warning message saying there is unreadable content in the document. It allows me to open the document anyway and doing a comparison with the original it says its the same. If I open the document from the email that is sent then its fine.
Not a big deal for me, but could be worrying for another user. I'm using CiviCRM 5.15.1 (and WordPress, but doubt that is relevant).