Hello.
Your question is in section extensions, so do you upload the file with any extension?
If you are using default import of Magento, you have to make sure that file encoding is UTF-8 without BOM and that you have included all the mandatory for upload fields with correctly formatted data.
Check the list of required fields with info -
https://www.mag-manager.com/useful-articles/tipstricks/required-csv-file-values-for-error-free-magen...