It's not very clear what you're asking for.
There is no generic method to find out which applications have which files loaded. Every application is different and may or may not expose this information at all. Most don't.
Now, if you're asking to see which files a process currently has open (for read and/or write, NOT "loaded"), then you have to enumerate the handles of the process. This requires administrative priv's. You can read about what it takes to do this
here[
^].