If we need to give students a file via the assignment module we would upload
it to the course files, take a note of the url location for that file, and then add a link to it in the html assignment description section. We end up with:
View this document</a>
which you can get by going into the course files and right-clicking on the excel file and choosing properties to get the address. If you also find the address of your excel icon file you could include that to keep the resource appearance of the site.
If you use the exercise or workshop modules I believe they use a tutor uploaded file rather than a html description to set assignments.