Allowing all/different file type uploads

I had the same issue, it looks like WordPress isn’t able to determine the correct MIME type for RFA files, so it defaults to:

'application/CDFV2-unknown'

Changing the MIME type to this fixed it for me. So it would be:

add_filter( 'upload_mimes', function ( $mime_types ) {
    $mime_types['rfa'] = 'application/CDFV2-unknown';
    return $mime_types;
} );