Why does excel Worksheets that are being attached to email to me changing to zip files?

One of my business associates has sent me 2 different emails with Excel attachments.  When she sends them from her they are just file attachments.  But when I receive them they tell me to "download as a zip file" and I cannot open like I have always done.  She did not send them as a zip file, she does not even use zip files.  Once I download it as a zip then it has compatibilities issues when I save it, telling me it may not be able to save it all. It never did before.  I work with these files every month and I had no trouble until a week or so ago.  Help, please.