Insert Extension into Filenames

Now this is a fun project! I have 3,000 image files (all jpg) that do not have extensions on them... they are all between 5 and 50K in size. I will provide a zip with the files (25 MB in size) - I ask that the provider return the zip to me with the (.jpg) extension appended to all filenames. I am sure there is some batch program that I can run to do this, but I have not been able to find one.