The data with a .twb files is a live connection. Either extract the file into Tableau or go back to the Excel side and modify all the fields there. The field likely contains string data and so Tableau is seeing it as a String by default instead of throwing data away.
I tried both extracting the data into Tableau (after selecting the correct data type) and modifying the data types within the Excel file. No luck on either. I have handed this report off and have instructed the user that he will need to manually update a couple of fields each time, unfortunately.
Thanks for your response!