"Untitled attachment" on emails?

Sending emails to a client today from my Macbook I encountered an issue I've never seen - and want to be sure I'm not infected with a virus. Client is on a PC with Microsoft Outlook and gets my email this morning with a Word fille attached, plus an icon that says "Untitled attachment 00548" and appears to be a blank html file. It's never happened before, and we send files back and forth often. Is this a virus coming from my Mac, or some other kind of glitch?