The listFiles() method will use the default File class, not your extended WallPaper class. Therefore you will get an error if you try to cast one of those objects to a WallPaper. Instead, do this:
String [] nameArray = wallFile.list(new TextFileFilter());
File [] fileArray = new File [nameArray.length];
for (int i=0;i<nameArray.length;i++)
fileArray[i] = new WallPaper(new File(wallFile, nameArray[i]).getAbsolutePath());//provided you have a WallPaper constructor that accepts an absolute path