Allow data image type for the main.js
Allow data image type for the main.js

Relates to #194

GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app,GoogleCloudPlatform/woolaroo-language-learning-app