How can I find the correct extension for a file without one?

I have a large number of files with no extension on my android device. I do not know how I got them, probably by downloading. If, from the file name and date, I can guess what extension they should have, I can usually get them to open by adding that extension manually. Does anyone know of an app, or a command, that examines a file and returns a filetype? Ideally one that can then append the correct extension. I believe most of these files are probably ebooks, PDFs, or some kind of media. If any of them are actually exes, I want to delete them, not run them.