Finding an image

I'm not that good with php yet, so please try to make it simple :)

I've got an uploader that will upload jpg's and gif's to the users directories. The problem is that I don't know how to find out which type of file has been uploaded there (jpg or gif) when I want to display that image. My question is, how can I find out the extension in the directory and then echo it as $username.gif or $username.jpg

Sorry it's not very clear.