While you can certainly rename a file to insert any extension, the results are less than ideal if you use an incompatible extension: the file will not open as expected. For instance, if you use an image file extension for an audio file, you will encounter an error when you try to open the file.
Even with a compatible file extension, expect to see a few errors occasionally if you simply rename the extension. Your best bet is to use a file converter for flawless conversion.