This afternoon I received an email with an attachment from a co-worker. She happens to sit across the aisle from me and, before I’d even realized I had an email to look at, she said, “That file won’t open for some reason, but it opens in Excel.”
Curious, I went to see what she was talking about. Here is an (edited) shot of the header of that email:
Email header from Outlook, showing the attachment.
Did you notice the generic-looking icon on that attachment? This is the primary clue to what’s wrong with this file: it doesn’t know what program it belongs to.