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function add_custom_upload_mimes( $existing_mimes ) {

   // add files to the list of already existing mime types

    return array_merge( $existing_mimes,

        array(

            'svg' => 'image/svg+xml',

            'rfa' => 'application/octet-stream',

            'rvt' => 'application/octet-stream',

            'pcg' => 'application/octet-stream',

            '3dm' => 'application/octet-stream',

            '3dx' => 'application/octet-stream',

            'skp' => 'application/vnd.sketchup.skp'

            )

        );

    // return the array back to the function with our added mime type

    return $existing_mimes;

}

add_filter( 'mime_types', 'add_custom_upload_mimes' );