The latest version of the mime types document here:
https://www.iana.org/assignments/media-types/media-types.xml
Does not list e.g. image/jpeg or image/gif, so our diagnostics flag these as wrong. It's not clear to me whether the omission is erroneous or not, but either way, it appears that we can no longer trust that document to provide a reliable list of mime types for checking purposes. We should revisit this in a week or two to see if it was an error that got corrected (the current version is only a few days old).