The criteria you've described looks a little too vague for a straight answer.
What webmail are you using - what computer and OS do you have? What version of excel file is it? And do you have Microsoft Office on your mac? Where did the "original" or source file come from (OS and software version)
My vague answer could be: If you don't have Microsoft Office on your computer - perhaps it's using "text edit" or Adobe to open the Word files. (which gets presented as basic text, or some other non-word format.) I don't think Apple OSX supports .xls files natively. (though can't say for sure - I've had word on this Apple since day-1)
Another possibility could be the origin of the excel file - some companies interested in protecting intellectual property will prevent export of files that could be used to populate a large database with hijacked information. They will block the attachments from being launched WAN-side.
If the excel file was large - it may have exceeded your "file size cap" for your web-based email. (if XXX webmail allows up to 10 megs for email attachments, and your spreadsheet was 15 - you won't get the attachment.