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Yep, there is.

See: http://www.javaprogrammingforums.com/java-code-snippets-tutorials/4954-java-tipe-aug-4-2010-how-use-file-filters.html
However, I'm not sure if it will automatically add the...

Simply modify the file object you get back:

File f = c.getSelectedFile();
String filePath = f.getPath();
if(!filePath.toLowerCase().endsWith(".txt"))
{
f = new File(filePath + ".txt");
}...