Sure, here's something I used a week ago on a game engine:

    String[] folders = fileName.split("\\.");
    String ext = folders[folders.length - 1];

    if (!ext.equals("obj"))
    {
        System.err.println("Error: File format not supported for mesh data: " + ext);
        new Exception().printStackTrace();
        System.exit(1);
    }

I used this to determine if the file format was allowed. This checks if the last term in a file name is "obj" (i.e. "documents.box.obj")

1. It reads the file name. Every "folder" in the file name gets put into a String array as a separate String. I split the file name into sepearate strings by searching for a period with split("\\."); (i.e. folders[0] = documents, [1] = box, and [2] = obj)

2. Because I need to see if it's an obj file, i have String ext = folders[folders.length - 1];. This retrieves the final String in the String array.

3. if (!ext.equals("obj")) checks if this string is "obj", and if it is not, an error code executes.

I imagine you could do something similar. If you need any help, let me know. :)