No, I never get to the mapping step. I’ve adhered carefully to the CSV specifications, read the FAQ before I even started, double-checked the character encoding, and just to be sure, I first moved my data to the header file you provide. I tried a couple of variations on the CSV file, including the csv/txt rename, and I also tried importing both with and without the Disable Real MIME Check plug-in. But nada, nothing, zip, beyond the “not permitted for security reasons” error.
I’ve done many CSV uploads for a differing plugin, WP Google Map Pro, so I’m comfortable with the drill.