Array( [error] => Sorry, this file type is not permitted for security reasons

This issue has been caused as a result of a WP security change and it could be happening on your site for a number of different reasons (WP version, PHP extensions, Multisite rules, the program used to make the CSV file), none of which are as a result of any of our code, there are several WP bugs as a result of this
https://core.trac.wordpress.org/ticket/45615
https://core.trac.wordpress.org/ticket/45633
(and many many more)

Almost every plugin that accepts CSV uploads was affected by this, you will find plugins such as WooCommerce and Easy Digital downloads docs mention how to fix it and many reference that plugin.

At this point from your WP.org review and its multipel changes, i think you might have realised this?

We treat our customers as they treat us, you be nice and we will be nice.

Stiofan

Lets move forward…

As i mentioned there are many different reasons it might be showing so without details we can’t provide a specific solution.

You can simply add the plugins code to your theme functions file or use the plugin “code snippets” to add it.

function wp39550_disable_real_mime_check( $data, $file, $filename, $mimes ) {
	$wp_filetype = wp_check_filetype( $filename, $mimes );

	$ext = $wp_filetype['ext'];
	$type = $wp_filetype['type'];
	$proper_filename = $data['proper_filename'];

	return compact( 'ext', 'type', 'proper_filename' );
}
add_filter( 'wp_check_filetype_and_ext', 'wp39550_disable_real_mime_check', 10, 4 );

Thanks,

Stiofan