Hello,
I tested importing a .csv file and did not encounter the error with the latest version of the plugin and WordPress.
You would normally get this error if WordPress was not allowed to receive this file type. The plugin normally uses the WordPress upload_mimes filter to white-list CSV files. Some thoughts:
1. Are you certain that you are uploading a csv file? Perhaps the file has picked up another extension by accident? Please check.
2. The upload might be blocked by some other plugin, especially security plugins. Does the upload succeed if you deactivate other plugins?
3. As a workaround, you could try adding define(‘ALLOW_UNFILTERED_UPLOADS’, true); to your wp-config.php file. If you do this you should be able to import your transactions, but you should then remove this constant again, for security reasons.
Hope the above helps, but please let me know if it doesn’t.
with regards