I wondered that, too, but I couldn't find any HTML documentation that blessed one form as superior to the other. And I would be surprised if Apple automatically converted images for "accept=image/jpg" but not "accept=.jpg".
https://html.spec.whatwg.org/multipage/input.html#attr-input...
> Authors are encouraged to specify both any MIME types and any corresponding extensions when looking for data in a specific format.
> User agents should prevent the user from selecting files that are not accepted by one (or more) of these tokens.