I have done this kind before and it works:
Please follow the below steps:
1. Create a SP list custom list.
2. Add item and fill title with some name and attach the Excel Sheet Template. Next save the item.
3. Go back to your PowerApp and add the above SP list connection.
4. Now go to the location where you want icon/button and insert Forms and select Edit.
5. Connect this form to the new SP list as data source.
6. On the form remove everything except the attachment datacard.
7. In the Form Items property add this: LookUp(*Your SPlist*,ID=1)
8. note....We are using ID=1 because that is the only record/item in that list, change the value ..... you know what i am talking about.
9. Modify this datacard to look good and pretty. Add a label inside saying Click to download etc etc..
10. You are pretty much done
Something like this.
