Hey Sean,
Your request is just a little ambiguous.
If you haven't read this it's worth a couple of minutes of your time.
Tip: How Do I Get The Best Answer in the Shortest Time?
What do you mean a certain file?
What differentiates it/them? Just the file extension?
How many different possibilities are there?
It looks like you're going to want to start with a Folder trigger.
There's not a lot of information in the wiki, but there's quite a bit of discussion on the forum.
Do a little research and report back if you get stuck.
-Chris