private static function getFilenameFromUrl(url:String):String
{
	var letters:Array= [];
	var charPosition:int= url.length;
	var currentCharacter:String;

	while(currentCharacter != '/' && currentCharacter != '\\') {
		currentCharacter = url.charAt(charPosition);
		letters.push( currentCharacter);
		charPosition--;
	}
	letters = letters.reverse();
	letters.shift();
	var filename:String = letters.join('');
	return filename;
}

Retrieves a filename from a complete url

Flash: Get filename from full url

file

ActionScript 3