Most likely it has to do with the way the program your sister-in-law uses to send you mail and how it encodes the email attachment. It's easy to forget these days that the official email specification is for PLAINT TEXT and PLAIN TEXT ONLY. No attachments, no formatting beyond capital letters, just plain text. Everything beyond plain text is outside of the official specification and so is no longer guaranteed to work because everyone can choose to implement additional features in whatever way they want. So the way your sister-in-law's email client is encoding attachments is clearly in a manner that WLM can't interpret. Yahoo and Google have clearly implemented code for handling such attachment types, but Microsoft has not. So you can either live with it or change email clients.