Recognize image/jpg as MimeType::Jpeg

[alissonlauffer]

With this patch, when MIME type is image/jpg, Lofty will recognize it as MimeType::Jpeg too, instead of falling back to Unknown.

I tested this patch with Amberol (Which uses MimeType:: enums to decide whether to show album arts) and now it displays JPG album arts just fine.

In my opinion, I don't see a reason for creating a MimeType::Jpg enum, since actually JPG and JPEG are the same thing but with a different extension.

[Serial-ATA]

Thanks!