Batch configurations don't automatically contain mappings for all default extensions

Steps to reproduce:

• Open a clean Rainbow session, create a pipeline, and export it as a batch configuration.

The batch configuration will contain default filter mappings for a number of extensions, but others will be missing. For example, .docx is correctly mapped to okf_openxml, but .xlsx and .pptx are not mapped to anything! This happens despite the fact that Rainbow itself maps all three of these extensions to the default okf_openxml filter configuration.

The problem is that the filter configurations defined by filters use a semicolon syntax to bind to multiple extensions (for example, ".docx;.xlsx;.pptx"), but the code in BatchConfiguration.exportConfiguration() that generates the list of mappings ignores all extensions after the first.