JPEG vs JPG: the same image format

JPG and JPEG are two filename extensions for the same image format. Changing one to the other does not convert or improve the image.

image/jpeg

Both extensions use the image/jpeg MIME type. If an upload form insists on .jpg, you can rename a genuine JPEG file from .jpeg to .jpg. Renaming a PNG to .jpg does not turn it into a JPEG: use a real conversion tool for that.