Ok, if you are getting the “ERROR: Sorry, this file type is not permitted for security reasons.” message, this is a WordPress error message not Connections.
WordPress 4.7.1 introduced a change which is causing many users trouble with many plugins which allow uploading. This error even shows itself in the WordPress Media Library. You can read the gory details in the WordPress Trac (not really necessary).
I know you said you downloaded the latest CSV Import this morning, can you confirm the version number that is installed?
In the WordPress Trac they recommend installing the following plugin as a workaround:
But this should not be needed with the latest version of CSV Import. But if you are certain you do have the latest, then I recommend giving the Disable Real MIME Check plugin a try.