@ James
The security update of WordPress, version, 4.7.1, broke file uploading. This has affected a lot of plugins. As you can imagine.
The WordPress dev are aware of this issue and are working to fix it with 4.7.2. The WordPress Trac ticket is here:
https://core.trac.wordpress.org/ticket/39550
Until they have a fix and release it, they are recommending installing this plugin:
https://wordpress.org/plugins/disable-real-mime-check/
Can you please install and activate this plugin and then try to import your CSV file? It should allow you to upload CSV files until the WP dev release 4.7.2.
I thought they would release a fix by now. But it does not seem that high priority for the WP devs (pure speculation). So I’m debating on releasing an update to CSV Upload that implement the workaround instead of waiting to WP 4.7.2.
