Currently it's only possible to select one field at a time. You have three options to download multiple attachment fields:
Use our Merge Airtable Attachment Fields automation to first merge your target fields into one, and then set the Bulk Download Attachments automation up to download the merged field.
Set up multiple instances of the Bulk Download Attachments automation, each to download a specific field.
Bulk download one field at a time, switching the selected fields after each run.