This is most likely because the sender's email software is configured to send their e-mails in Rich Text Format. Rich Text Format affects attachments they send with the message, even if they were created by Excel or Word.
Some e-mail programs that are not in the Microsoft Exchange family cannot convert those incoming attachments properly. The best way is to ask your sender to configure to send messages in Plain Text. They can select to send via Plain Text under the Mail Format tab.