Do you have any app installed which can handle Excel files? If yes, from within the mail App, open (view) the excel file - in the upper right corner is the (let's call it) "export-" button - tap it and "open in" that App.
There are lots of free (and paid) office apps around which can manage excel - anyway, they will not save the file into a folder as known from PC.
If you just need to store the file you may use cloud service with App - like box.net or dropbox (just 2 examples)...