gatsby: BUG: MozJPEG with PNG and `toFormat: JPG` outputs PNG with jpg extension

Description

When enabling mozjpeg in my gatsby-config.js, jpg images do see better compression, but any png source images if using toFormat: JPG in their graphql query, will not actually format the image into jpeg, but instead output a png image data with a jpg file extension.

Steps to reproduce

Enable mozjpeg in gatsby-config.js for gatsby-plugin-sharp:

//   plugins: [
    {
      resolve: `gatsby-plugin-sharp`,
      options: {
        useMozJpeg: true,
      },
    },

Use the default starter project with gatsby new and edit the image.js component to include toFormat: JPG:

const Image = () => (
  <StaticQuery
    query={graphql`
      query {
        placeholderImage: file(relativePath: { eq: "gatsby-astronaut.png" }) {
          childImageSharp {
            fluid(maxWidth: 300 toFormat: JPG) {
              ...GatsbyImageSharpFluid
            }
          }
        }
      }
    `}
    render={data => <Img fluid={data.placeholderImage.childImageSharp.fluid} />}
  />
)

Build the site, notice no error, check the image assets, eg the same dimension will be output 800x800(despite the maxWidth: 300?) it is roughly 150KiB, vs about 34KiB when mozjpeg is not enabled. For the default displayed image of 300x300, it’s 9KiB vs 40KiB(mozjpeg). It displays fine in the browser so can go unnoticed, if you use any image tools or for some file browsers(Dolphin for me) the image preview may fail to generate because it’s not actually a jpeg image when inspected.

If unable to reproduce, this could be due to the build of mozjpeg installed to your system/OS. Some mozjpeg will support PNG input if they’ve been built with that support, mine does not. I can provide a reproducible project with an Alpine docker container if interested.

Expected result

Either actually convert into a JPG image format, or don’t use jpg extension(and probably throw an error/warning instead of failing silently).

Actual result

PNG image format but jpg file extension. PNG did have slightly reduced filesize for the same dimensions but was still PNG data.

Environment

Gatsby v2.4.8, Custom Alpine Linux 3.8 Docker container, NodeJS v10, custom built mozjpeg(v3.4.0 via CMake not autotools, functions correctly for JPG, only static and win32 optionally support PNG via CMake builds). Project is the default starter from gatsby new.