If the .csv file was created on a Mac, it will not upload. Saving a .csv on a Mac encodes special characters that come out garbled between the Mac and the import. Mac also creates 'line endings' issues that can be interpreted as line feeds or carriage returns in the CSV file.
The solution? When creating CSV files in Excel, on a Mac, use the option to Save As “CSV (Windows) ,” otherwise the format will not be correctly recognized by .csv Import or just save the file on a PC or open the CSV files on a PC.