MDX File Extensions

By default, only files with the .mdx file extension are treated as MDX when using the gatsby-source-filesystem. To use .md or other file extensions, you can define an array of file extensions in the gatsby-mdx section of your gatsby-config.js.

module.exports = {
plugins: [
{
resolve: `gatsby-mdx`,
options: {
extensions: [".mdx", ".md"]
}
}
]
};