CSV files are preferred and return the best results. Tab delimited text files with a .txt extension are also accepted.
To minimize upload time, you can zip compress large files prior to uploading them. Zip archives should only contain a single file with a .csv or .txt extension.
For regular uploads, files should have a minimum of 25 records. Files uploaded through the API require only 1 record.
Learn more: How should I format my file for uploading?