var files = Directory.GetFiles(@"C:DownloadsTemptest");
foreach (var file in files)
{
  if(file.Contains(".gif"))
  {
    string testing = string.Format("{0}.png", Path.GetFileNameWithoutExtension(file.Replace(".gif", "")));
    File.Move(file, @"C:DownloadsTemptest"+testing);
  }
}

var files = new DirectoryInfo(@"c:files").GetFiles(@"*.gif");
foreach (var file in files)
    File.Move(file.FullName, string.Format("{0}.gif", Path.GetFileNameWithoutExtension(file.FullName)));

var files = Directory.GetFiles(@"c:files", @"*.gif");
foreach (var file in files)
    File.Move(file, string.Format("{0}.gif", Path.GetFileNameWithoutExtension(file)));

REM first rename all .gif.png to .gif
for %i in (*.gif.png) do ren %i %~ni

REN then rename all .gif to .png
for %i in (*.gif) do ren %~ni.gif %~ni.png

import java.io.File;
import java.io.FilenameFilter;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main
{
    public static void main(final String[] argv)
    {
        final File   dir;
        final File[] entries;

        dir     = new File(argv[0]);
        entries = dir.listFiles(new FilePatternFilter(".*\.gif\.png"));

        for(final File entry : entries)
        {
            if(entry.isFile())
            {
                final String path;
                final String newPath;
                final File   newEntry;

                path     = entry.getPath();
                newPath  = path.substring(0, path.length() - "gif.png".length()) + "png";
                newEntry = new File(newPath);

                if(!(entry.renameTo(newEntry)))
                {
                    System.err.println("could not rename " + entry.getPath() +
                                       " to " + newEntry.getPath());
                }
            }
        }
    }
}

@echo off
setlocal enabledelayedexpansion enableextensions
for %%f in (*.gif.png) do (
    set fn=%%f
    set fn=!fn:.gif=!
    ren "%%f" "!fn!"
)
endlocal