"Jpeg" is the name of the imaging format. The ending of such a formatted image is "jpg."

Files ought to end with ".jpg"—though some platforms will accept "jpeg."

To make a comparison, I work in Photoshop nearly daily. All of my Photoshop documents end with ".psd." That is the Photoshop ending. One would not call a document, "xxxxx.photoshop." that would be silly. It's the same thing with Jpegs.