We support .csv, .xls, and .xlsx files, formatted in Unicode (UTF-8). When exporting a .csv file make sure text encoding is set to Unicode (UTF-8).
If you are trying to upload a file and you see an error stating that the file you've provided is in a wrong format, try to open that file with either Apple Numbers or Microsoft Excel and export or save that file again in the Unicode (UTF-8) format. Retry to upload the file.
Please contact support if the error is persisting, and we'll help you troubleshoot.