Support bundle with extension

Bundle with extension in its name will has API response served
directly instead of the standard JSON response.

Currently, the supported extensions are `.kml`, `.xml`, `.json`.

Note that when using this type of bundle, only the first key in
the bundle will be served.

For example, by exporting `sample.kml` instead of just `sample`,
the first API's response will be returned when performing request
to `/sample.kml`.