It looks to be URL encoded with utf-8 charset. You can take cue from the following sample conversion:
System.out.println(URLEncoder.encode("лл.txt", "utf-8")); // Gives %D0%BB%D0%BB.txt
System.out.println(URLDecoder.decode("%D0%BB%D0%BB.txt", "utf-8")); // Gives лл.txt