AppEngine Static File MIME Type Override

In AppEngine, static file handlers can occasionally override the correct MIME type, resulting in the error message: "Could not guess mimetype for [file]. Using application/octet-stream..."

Problem Resolution

To correct this issue and specify the correct MIME type for static files, follow these steps:

1. Check Local MIME Type Definition: On your local machine, the correct MIME type for the file extension may be defined in the /etc/mime.types file. AppEngine, however, may not have access to this definition.
2. Use "mime_type" instead of "http_headers": In your app.yaml configuration file, change the "http_headers" directive to "mime_type" for the problematic file extension. For example, to specify the MIME type for .otf files:

- url: /home/font/(.*\.otf)
  static_files: home/font/\1
  upload: home/font/(.*\.otf)
  mime_type: application/x-font-otf

Example App.yaml Configuration

The following app.yaml configuration demonstrates the correct use of "mime_type":

application: test
version: 0
runtime: go
...

handlers:
 - url: /home/font/(.*\.otf)
   static_files: home/font/\1
   upload: home/font/(.*\.otf)
   mime_type: application/x-font-otf
...

Additional Resources

• AppEngine documentation: https://developers.google.com/appengine/docs/python/config/appconfig#Python_app_yaml_Static_file_handlers