I get files with random extensions sometimes in order to bypass corporate firewalls. Usually I would just ask the other party what it is that I just received. Other times I get curious and open the file with notepad. Sometimes the file would have a plain text header which describes how to open it. Could this potentially cause "issues" if I didn't do it in a sandbox?
In theory, someone might be able to find a buffer overflow in notepad that could cause issues when opening a file, but the chances are pretty minimal and the file would have to be specifically made to exploit notepad. A sandbox is still technically the safest, but I generally view files in Total Commander's viewer (which is equivalent to opening it in notepad) because the risk is so low that it isn't really a problem unless you have really high security needs.