e-/exhentai's original image downloads currently send
incomplete/invalid Content-Type headers, "jpg" instead
of "image/jpg" etc, since the last update.
(https://forums.e-hentai.org/index.php?showtopic=236113)
This change prepends any Content-Type value missing a
media type specification with "image/", transforming it
into a valid MIME type.
(A global solution to a local problem, but it shouldn't
cause any issues anywhere else)
115fd2c6f2