HI!

It's bug of this plugin, author must fix it soon or a workaround would be to temporarily add this to wp-config.php:

define('ALLOW_UNFILTERED_UPLOADS', true);

And temporarily remove or comment out this block in wp-content/plugins/widget-importer-exporter/includes/import.php:

    // Check file type
    // This will also fire if no file uploaded
    $wp_filetype = wp_check_filetype_and_ext( $uploaded_file['tmp_name'], $uploaded_file['name'], false );
    if ( 'wie' != $wp_filetype['ext'] && ! wp_match_mime_types( 'wie', $wp_filetype['type'] ) ) {
      wp_die(
        wp_kses(
          __( 'You must upload a .wie file generated by this plugin.', 'widget-importer-exporter' ),
          array(
            'b' => array()
          )
        ),
        '',
        array( 'back_link' => true )
      );
    }