fix early bail when testing if string field should link to File node (g…

…atsbyjs#6504)

* fix early bail when for file inferring

* remove any `application/octet-stream` handling to be in line with mime@2 behaviour

* this can be shorter, doh!

src/create-file-node.js

@@ -44,7 +44,7 @@ exports.createFileNode = async (
     internal = {
       contentDigest,
       type: `File`,
-      mediaType: mediaType ? mediaType : `application/octet-stream`,
+      mediaType,
       description: `File "${path.relative(process.cwd(), slashed)}"`,
     }
   }