var directory = new DirectoryInfo(@"C:\");

var latest = (from f in directory.EnumerateFiles("*.jpg")
orderby f.CreationTime
select f.FullName).LastOrDefault();

Console.WriteLine(latest);

Returns null in case there are no jpg files found.

var directory = new DirectoryInfo(@"F:\Root\");

var files = from f in directory.EnumerateFiles("*.dat")
group f by f.Name.Substring(0, 21) into g
select g.Last();

foreach (var file in files)
{
Console.WriteLine(file.Name);
}