[schema] Fix error when guessing preview config for files

bjoerge · bjoerge · commit 7fd1e1f5457f · 2017-09-01T11:52:20.000+02:00
diff --git a/packages/@sanity/schema/src/types/file.js b/packages/@sanity/schema/src/types/file.js
@@ -53,7 +53,7 @@ export const FileType = {
       })
     })
 
-    lazyGetter(parsed, 'preview', createPreviewGetter(subTypeDef))
+    lazyGetter(parsed, 'preview', createPreviewGetter(Object.assign({}, subTypeDef, {fields})))
 
     return subtype(parsed)
 
