changed filenameExtension from String to array in data_format schema. This might introduce braking changes (easy to fix)

closes #17 (closed) Changed filenameExtension from String to array in data_format schema.

@Akhil.Patil This will probably brake validation for all previous data_format records which filled this property with a string. These could changed via script to single element arrays.

I also changed the description to ask for preceding "dots" but have not implemented any enforcement logic (possible through a regex pattern).