diff --git a/packages/gatsby-plugin-mdx/gatsby/source-nodes.js b/packages/gatsby-plugin-mdx/gatsby/source-nodes.js
index b3030f492dacd..249b626728383 100644
--- a/packages/gatsby-plugin-mdx/gatsby/source-nodes.js
+++ b/packages/gatsby-plugin-mdx/gatsby/source-nodes.js
@@ -269,6 +269,11 @@ ${e}`
       },
     },
     interfaces: [`Node`],
+    extensions: {
+      childOf: {
+        mimeTypes: options.mediaTypes,
+      },
+    },
   })
   createTypes(MdxType)
 }