Change "filenameExtension" of a data format to type "array" and improve property description

1. In hmc_data_format_1_0_0.json the property filenameExtension is of type string. But for many data formats there does not exist "the one and only single extension of truth". Very often multiple extensions are common, see the examples of HDF or KML. It might be feasible to change this property to an array of string in order to be able to specify different valid extensions.

2. The current description of filenameExtension – "A typical filename extension for the data format." – does not specify if the extension is to be written with or without the prefixing dot: E.g. .kml vs. kml. What was the initial intention? With or without dot? Or is this irrelevant?