Source pattern with no extension will not traverse all files

[senica]

preprocess:
    build:
        options:
            inline: true
            context:
                ENV: 'dev'
        src: ['build/**/*']

Will give a file not found error

However, if I do src: ['build/**/*.js'] then it's all good.

Not sure if this is a bug or if it just needs to specified in the documentation.