one solution could be using Datetime stamp(with your desired datetime format) in the file name like :
test201406170449.txt ; test201406170450.txt ;test201406170451.txt ; ...
another one could be using numbers at the end of the filename for example:
test.txt ; test1.txt ;test2.txt ; ...
But if you want to keep only changed ones you'd better use file's lastupdateDate property in order to check if the file has been changed since the last upload or download.
FileInfo fi = new FileInfo("FilePath");
DateTime lastUpdateDate = fi.LastWriteTime;