Are you uploading on FRONT end form or BACKEND form?
Did you set "Allowed File Types" in Settings for "Private Files"?
This should be a comma separated list od extensions like "_txt,doc,docx,xls".
I uploaded a spreadsheet and downloaded it without any issue using the FRONT end form. The file extension was "xlsx".
I thought that I recently saw an October CMS config setting for file types but can not recall at the moment. I will search for it and post a link if I find more.
We are proud to announce that the stable release of the October CMS 2 is now available! The release represents over a year of design, research and development effort, which resulted in multiple new features and changes.