For hot folder functionality Express Data Prep Wizard supports CSV (comma separated value) and Excel data files. You need to provide one within the wizard so that desired format (header) can be recognized. No real data is required to be in this file. Just column names.
Later on, when you upload your data files to the incoming hot folder, all those files have to follow exactly the same format (header). Otherwise the job execution fails.
If you do not need to use hot folder functionality many other data formats/sources can be used. See Connecting to your datastore and/or Express Data Prep Wizzard -- Input&output sections for more details.