function check_file_format( file_format ) {
	if( file_format.match(/jpg/i) ){ return true; }
	if( file_format.match(/jpeg/i) ){ return true; }
	if( file_format.match(/gif/i) ){ return true; }
	if( file_format.match(/png/i) ){ return true; }
	return false;
}

function show_error( error_id, msg ) {
	var error_msg = document.getElementById( error_id );
	if( error_msg ) { error_msg.innerHTML = msg; error_msg.style.visibility = 'visible'; }
}

function hide_error ( error_id ) {
	var error_msg = document.getElementById( error_id );
	if( error_msg ) { error_msg.innerHTML = ''; error_msg.style.visibility = 'hidden'; }
}

function show_element ( element_id ) {
	var element = document.getElementById( element_id );	
	if( element.style.display == 'none' ) {
		element.style.display = 'block';
//		element.style.visibility = 'visible';
	} else {
		element.style.display = 'none';
//		element.style.visibility = 'hidden';
	}
}

function check_email_form( sender, recip ) {
	var sender_obj = document.getElementById( sender );
	var recip_obj = document.getElementById( recip );

	if( sender_obj && ! sender_obj.value ) {
		show_error( 'email_form_error', 'Please, enter sender data' );
		return false;
	}

	if( recip_obj && ! recip_obj.value ) {
		show_error( 'email_form_error', 'Please, enter recipient data' );
		return false;
	}

	return true;
}