Assume a situation when you want to change extension of multiple files in a folder at once. For example you have received a bunch of log files which is not associated with any program in your system, these files do have an extension .log. One simple idea could be to rename all files into .txt and open it into any text editor to see the content.
To rename all files in a single command Just issue below command from DOS window:
ren *.log *.txt
You can see in the below example all files with .log extension has been renamed to .txt. This trick should work on all versions of Windows.
You may also do a similar trick to combine multiple text files into one from command line. Find the details here.