Actionscript：如何确定某些加载内容的类型？

I have a situation where I'm loading some content using a URLLoader but the content could either be a video (flv or swf) an image(jpg or png or gif) or a swf. The trick is, I need to react in different ways based on the type of content that's loaded without necessarily knowing what type of content it is beforehand.I have a situation where I'm loading some conte