Invalid File Upload when using DataBridge

Overview

You are using a profile in DataBridge and have selected to import a txt file. 
Now you want to change the file in the profile to an xls file but DataBridge tells you this is not allowed, why is this?

How To

The reason is that txt and xls files use different connectors in DataBridge.
CSV connector = txt, csv, tsv
Excel connector = xlsx, xlsm, xltx, xltm, xls, ods

When you change the file, you see that in this example below from Dropbox, you cannot select the other file type.

With a manual upload, you will receive a message that the file is invalid.