The only thing I can think is that your file (which is an Excel file) is not saved in the proper format.
I tried to duplicate your problem - and I did. But, to do so, I saved the file as a "Microsoft Excel 5.0/95 Workbook." It ended in the .xls format and looked just fine. But, like you, I couldn't view the file in Mail, nor would any of my installed apps open the file. Very odd - because the iPad documentation says it can read .xls files.
This was by accident because I was trying to save in any .xls format - didn't know there were two available. Turns out there is a more updated version of the .xls file system - "Excel 97-2003 Workbook."
The latter .xls file does open in the Mail app viewing window. It opens in all my apps, also.
So, the only thing I can recommend is either (1) ask the person who sent you that Excel file to re-save it in the more current version (the Excel 97-2003 Workbook) or (2) save that file to a computer, open it with Excel and re-save it yourself in the most current version.
Bottom line: the iPad will not open that old .xls file. It'll have to be converted to newer in order to be viewed on the iPad.
Hope this helps.
Marilyn