Fixed lint warning: case converstion without a locale.

briar-core/src/org/briarproject/plugins/file/FilePlugin.java

@@ -9,6 +9,7 @@ import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
 import java.util.Collection;
+import java.util.Locale;
 import java.util.concurrent.Executor;
 import java.util.logging.Logger;
 
@@ -77,7 +78,7 @@ public abstract class FilePlugin implements SimplexPlugin {
 
 	// Package access for testing
 	boolean isPossibleConnectionFilename(String filename) {
-		return filename.toLowerCase().matches("[a-z]{8}\\.dat");
+		return filename.toLowerCase(Locale.US).matches("[a-z]{8}\\.dat");
 	}
 
 	private SimplexTransportWriter createWriter(String filename) {