.xlsm and .csv file extensions are not supported for file uploads on Nintex forms. This is frustrating because we use data files in this format that we want to upload via a Nintex form.
Users must save the files as a .xlsx file before they can upload files
Then when you use the form when going to select a file you'll only have the option of selecting a csv or xlsm file type.
https://i.imgur.com/Z79InVP.png
https://i.imgur.com/219yuIf.png