Deciding what content gets processed by gatsby-mdx. This is an advanced option that is useful for dealing with specialized generated content. It is not useful for most users.
mediaTypes: ["text/markdown", "text/x-markdown"],
Gatsby includes the media-type of the content on any given node. For
file nodes, we choose whether to process the content with MDX or not
by the file extension. For remote content or generated content, we
choose which nodes to process by looking at the media type.