Scanning Non-txt Files With Unspecified Filename

I want to make a tool that does the 2 following things::

- Open files that are not .txt but can be opened as .txt and return them as a string. It just returns an empty string at the moment.

- The filenames are unknown, just the file extension at the end and the the YYYYMMDD number in front are always the same, therefore I'd like the app to simply scan every file in the same folder (not the same file twice, obviously). How can this be done?

That's what I've got so far.

Java Code:

public String readFile(String filename) throws FileNotFoundException {
Scanner scanner = null;
File file = new File(filename);
String output = "";
try{

[Code] .....