Sorry, you are not allowed to upload this file type.

The error may arise from specific HTML tags in your .CSV file, which WordPress restricts for security purposes during the import process. However, you can bypass this restriction by adding the line 'define( 'ALLOW_UNFILTERED_UPLOADS', true );' to your wp-config.php file. 

These limitations are a fundamental aspect of WordPress, implemented to prevent potential security risks, such as the inadvertent uploading of harmful executable files that could compromise your website or server.