Update file-uploads.mdx (#694)

docs_src/src/pages/documentation/api_reference/file-uploads.mdx

@@ -40,7 +40,7 @@ Batman scaled his application across multiple cores for better performance. He u
 
       # write whatever filename
       with open('test.txt', 'wb') as f:
-          f.write(body)
+          f.write(file)
 
       return jsonify({'message': 'success'})
     ```