Fix 'Download All as Zip' Web UI action for a selection of documents

Download All as Zip action on a selection of documents of custom document type generates a ZIP file containing only the XML export, blob is missing.
How to reproduce:

• in Studio Modeler define a new document type:
  • Extends Document
  • Category Document
  • add schema file
  • define the Creation Layout for the JSF UI or the create layout in Studio Designer to allow the blob attachment
• Deploy in a Nuxeo instance
• create an instance of the custom document type and attach a blob in a Workspace
• using the Web UI, in the workspace where the document was created, select the created document and click on the selection action Download All as Zip

Expected result: the downloaded ZIP file contains the file attached to the document
Actual result: the downloaded ZIP file contains the XML export of the document