If you run WordPress with Multisite feature and tries to import reviews using our plugin, import might fail with an error message "The uploaded file is not a valid CSV file".
WordPress Multisite Settings
This error is likely caused by settings in your Network Admin area where it is possible to limit upload by file types. If the list of permitted file extensions doesn't include 'csv', then upload of the file with reviews will fail.
To solve this issue, you should go to My Sites > Network Admin > Settings and scroll down to the field Upload file types. Next, you should verify that the list of allowed file types includes 'csv'. If it doesn't include it, you should add it and save the settings.
If update of Network Admin settings doesn't resolve the issue, then you should check plugins installed on your WordPress site. One of them could be blocking upload of CSV files. For example, if you have "Enhanced Media Library" plugin installed, it could be blocking upload of CSV files. In this case, you should navigate to Settings > Media > MIME Types and adjust upload permissions to allow CSV uploads.