Fix incorrect MIME type from image/jpg to image/jpeg

Fix incorrect MIME type from image/jpg to image/jpeg

According to multiple sources, both official ([IANA] and [RFC2046]) and
otherwise (eg, [Mozilla] and [Wikipedia]), the official MIME type for
JPEG images is `image/jpeg`, not `image/jpg`.  Change Hugo to match.

[IANA]: https://www.iana.org/assignments/media-types/media-types.xhtml#image
[RFC2046]: https://tools.ietf.org/html/rfc2046
[Mozilla]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types
[Wikipedia]: https://en.wikipedia.org/wiki/JPEG